To our surprise the listed building people are happy for us to excavate our cellar floor, and put in foam glass insulation and limecrete - they have also approved using a drainage membrane which will hopefully channel any rising water to our flood pumps, and act as a radon barrier at the same time. However, it will then make the floor impermeable - and non breathable, so is there any point in putting limecrete on top of a plastic layer - and we will then possibly get back to the old problem of damp walls rather than a damp floor, as we have in our barn due to the concrete floor installed there some decades ago.

Anyone got any relevant experience to share?

That is what I'm thinking. We cannot tank the whole thing as it's 500 yrs old, listed, and looks amazing. It has flooded once, and the ground water rises and falls in the test pits we have dug, so we will be installing a proper flood sump, with two pumps in it, which I feel will get use in the wetter parts of the year. We have also dug a French drain at the rear, which also has pumps in it, and that may be why it's not flooded since. We have a (minor) radon issue, so a radon barrier appeals, but I think that ventilation is going to be important, so I'm putting a vent pipe in the lightwell at the front, and I'm seriously considering adding a fan to that as well so there is a constant flow of air outwards taking excess moisture and radon with it. We will also dig a radon sump, but suspect it may fill with water, but whilst we are digging, we may as well do it. We're also going to have a radiator down there, mainly to avoid cold air being sucked up the stairs into the main house. I think I'm going to go for a geotextile, foamglass, geogrid and limecrete construction, so the floor is totally breathable, and then hope the pumps will keep the actual water at bay, and accept that it's going to be a bit damp down there, as it is a cellar after all.

if you search posts I added some info on this subject, I was in a similar situation.

In brief, foamglass floats, so if you are at risk of groundwater they usually advise not to use it. I did, but you must ensure the weight on top of the insulation is enough to counteract buoyancy. i.e. I ended up with 300 mm foamglass + 120/150mm concrete on top

Foamglass keeps the building dry by draining down to ground, if you put an impermeable barrier underneath it's a bit pointless

French drain ++

I copied someone else who did a concrete island, i.e. you pour a concrete slab and leave a border between the slab and walls ( mine was about 300mm wide ), you then make up the border with something more breathable, e.g. expanded clay aggregate ( Leca / Lyapor etc.. ) + Lime/Sand mix. The floating risk here is reduced due to the friction between the border piece and wall and slab.

My own personal opinion after much reading, the whole limecrete is breathable, concrete is not is slightly controversial. I don't think it is as black and white as whether you use portland cement or HL5. HL5 has a similar mu to regular cement, although it certainly behaves better in harsh conditions than portland. The breathability of the floor is due to the microscopic pores, obviously Limecrete will be more breathable when mixed and laid by hand because it is not as compact. Concrete can be made more breathable by doing a bad quality mix, i.e. more water and no additives. I had to argue with the concrete companies to try and persuade them to bring me an old fashioned mix.. i.e. lime / portland cement + sand + stones, no additives and lots of water. The floor was bone dry within a few weeks and has remained so since, even after rising ground water last winter. And of course NO DPM, which would trap the moisture in the slab. Also I would make sure the concrete company do not substitute GGBS for cement and if you can get them to use a crushed limestone aggregate instead of pebbles that will also increase the permeability of the slab. I would love to say mine was perfect but I clearly p***edoff the 1st concrete company because they couldn't understand the logic, ignore me and delivered the concrete with GGBS in the mix, luckily we were doing a large area poured in bays and I managed to get what I wanted for the next couple of pours. So lots to consider, find a sympathetic concrete supplier.

What type of lime in your limecrete?
edit: and in your pointing? Also, what's its temperature?

I think the limecrete will have NHL3.5 in it. My 'coarse stuff' pointing has now hardened, but when I do the next phase I think I'm going to use NHL3.5 sharp sand mix for my mortar to speed things up a bit. Temperature is around 12c most of the time, going a bit lower at the moment, as it's not weather tight at the moment.