What for, exactly?

Keep it dry = more effective.

Same reason I want to run my DPM under my floor insulation:
http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=10141

Posted By: fostertomalmost negligible

Why would AECB recommend it then?

I've answered a bit more humbly, seeking expert confirmation, to this same question on http://www.aecb.net/forum/index.php/topic,3822.0.html

Oh found it here:
http://www.aecb.net/forum/index.php/topic,3822.0.html

LOL

Assuming we did tank them, what product would you suggest we use?

All true, so if you're on wintertime frequently saturated/high water table ground, then think again - affects both EPS and Leca insulativeness, but not completely, merely 'significantly'. So you may consider it acceptable, if for sufficient %age of time it can drain out again - mere wetness doesn't matter. If not, it's asking a lot to expect taNKING TO KEEP ALL PERMANENTLY DRY FOREVER, BECAUSE THE TINIEST FAILURE ANYWHERE WILL JUST MEAN An (sorry, cap lock) accumulation that can never dry out except maybe inward.

I'm wondering if the point of the AECB detail (which shows tanking of the EWI below DPC) is to avoid the need for a DPC joint through the insulation?

If it wasn't tanked wouldn't you want to continue the DPC through the insulation (which isn't shown on the AECB detail).

Posted By: fostertomEPS hardly wicks moisture so doesn't need a DPC thro it.

That's what my Visqueen guy said. In fact he said running a DPC through the EPS might actually be counterproductive in that moisture could be wicked along the DPC back to the structure.

Posted By: cc64Once I get past that stage I would intend to do a thermal skirt below dpc of XPS. My builder was sceptical that render to the outer face of that xps would be appropriate. He favoured a cementitious render called vandax ( sold by safeguard) applied to the sub dpc bricks before installation of the XPS. This he insisted would be more effective and, being behind the XPS, more durable.

I presume you mean Vandex? Not that I know anything about it. :(

But what is its purpose? You don't need it now, so why will you need it in the future? Unless you somehow make the floor of the undercroft waterproof, floodwater will find its way into the undercroft through its floor, and maybe find its way up into the house depending on how good and where the DPM is.

What's the strategy to protect the house itself from flooding?

edit to add: Oh and contrary to intuition I think EPS is better than XPS where it can get wet. And cheaper.

OK - strategy amended after a certain amount of reading.

One of the frustrations in a retrofit endeavour is the amount of knowledge now built-in to forum participants but which is no longer documented on the forum (usually because links point to expired resources). Just so in the near eternal debate on sub dpc EPS versus XPS.

From the property metrics of these two the XPS looks the no-brainer better; closed cell, better lambda than EPS. However the www is littered with sites claiming that contrary to expectations EPS is actually the better. These are frequently brought to you by EPS manufacturers and virtually universally reference a single study. Below a collection of such references:
https://www.wconline.com/articles/85794-eps-insulation-outperforms-xps-in-study # 2008!
https://www.concreteconstruction.net/how-to/site-prep/choosing-between-eps-and-xps-rigid-insulation_o
https://plastifab.wordpress.com/2019/08/13/the-great-debate-eps-vs-xps-for-below-grade-applications/
https://consupt.com/2014/05/the-dirt-on-below-grade-insulation/
https://www.insulfoam.com/below-grade-insulation-eps-vs-xps/
https://www.constructioncanada.net/sink-or-swim-a-comparison-of-below-grade-rigid-foam-insulation-moisture-performance/2/
https://www.sbcmag.info/news/2018/may/insulation-manufacturer-analyzes-cons-moisture#:~:text=An%20independent%20lab,R%2Dvalue%20Retention.%E2%80%9D # this last looks horrid but conveniently highlights said study as below.

An independent lab Stork Twin City Testing evaluated the moisture content of EPS and XPS buried side-by-side for 15 years on a building foundation in St. Paul, Minn. At the time the insulations were removed, the EPS had only 4.8 percent moisture by volume compared to 18.9 percent moisture content for the XPS. After 30 days of drying time, the EPS had dried to only 0.7 percent moisture by volume, while the XPS still contained 15.7 percent moisture.

The Stork Twin City Testing of the two insulations also showed the degree to which moisture absorption impacts R-value. The XPS insulation lost 48 percent of its R-value, compared to only a 6 percent decrease for EPS. For additional discussion of these results, see the EPS Industry Alliance’s Technical Bulletin 103(link is external), “15-Year In-situ Research Shows EPS Outperforms XPS in R-value Retention.” # note that EPS industry link is dead.

The fact that everyone refs back to a single study, for which I've found no original documentation, bothers me, and inevitably the words unconfirmed and uncorroborated come to mind.

But not so fast. While the www is littered with assertions that XPS is indeed the optimal sub grade insulant (often brought to you by XPS manufacturers), there is evidence to the contrary from OakRidge Lab study:ORNL/TM-2012/159 April 2012 'Measurement of Exterior Foundation Insulation to Assess Durability in Energy-Saving Performance'
https://info.ornl.gov/sites/publications/files/Pub36100.pdf. This compared sub (and supra) grade XPS performance with that of a fiberglass drainage board called W&D over a 15 year period from 1991. There's a great deal of nice work here - worth diving into - most of which I will not summarise here.

Both insulants gained moisture over the 15years of their installation. This moisture impacted R, but more modestly than suggested by e.g. https://www.constructioncanada.net/sink-or-swim-a-comparison-of-below-grade-rigid-foam-insulation-moisture-performance/2/ (which offered '''“Absorption of only 20 per cent moisture can cause up to 55 per cent loss of insulation value.” for some unspecified insulant)

XPS gained very much more moisture than the W&D, and this took disproportionately longer to dry out. The authors suggested "This can be explained by the different pore size structure of the two different insulation types (XPS and W&D). Both have approximately the same porosity, but it is distributed at an unequal pore size range. XPS has many more small-sized pores compared to the W&D fiberglass and hence has higher capillary suction forces. These capillary suction forces at smaller pore sizes are much higher than gravity and prevent moisture from draining out of the XPS to the water table. This is apparently possible for the moisture in W&D insulation as the larger pore sizes have only minor capillary suction pressure. The drying time for the XPS samples is slightly more than 3 months, which is much longer than the drying time for the W&D insulation. This is due to the lower moisture content as well as the higher vapor permeability of the W&D insulation, which enables a higher diffusion and evaporation rate."

The more open structure of the W&D insulant allowed it to dry-out/drain than did the nominally superior closed cell structure of the XPS. It seems this is a variant on the (I think 'established'?) truth that we cannot possibly hope to achieve a completely water-tight enclosure, so we'd be better off ensuring an exit route for that which inevitably does get in.

Although I have found no comparable study for EPS, a 2year study, mostly of EPS, https://web.ornl.gov/sci/buildings/conf-archive/2001%20B8%20papers/188_M_Swinton.pdf
Indicates a stable thermal performance, "sustained even during major rainstorms and winter thaws when the effects of water movement were apparent at the outer face of the insulation specimens, contrary to expectations that the R-value would decline under such circumstances, especially if water were to move through the insulation."

so thanks to Dave, EPS it is; probably BASF Neopore
And no tanking to the subdpc engineering brick; better leave this simply to dry out when needed.