I mixed CWI and EWI on a home 14 years ago , theyve rpeorted no negative effect.
With hyrid insulation (mixing internal external) xtratherm suggest its ok as long as the outer layer is the better insulator (would require further research)

I agree the beads seems the best cost effective solution.
If the external render fails EWI could be looked at then.
Internally you could attempt to improve airtightness by using a parge coat with hardwall backing plaster behind electrical outlets or perhaps expanding foam and also look at sealing around the perimeter of the walls , corners , ceiling/floor to wall junction.
Where joists meet the wall under floor boards is also another well know air leaky area

I vote for the beads as well.

Regarding the comments from the PU foam rep - well he would say that, probably not an unbiased opinion.

When contemplating EWI with a cavity wall - don't forget that the cavity has to be insulated as well otherwise EWI won't work, so as said above beads first then EWI if justified.

Thanks all. I will get thermal imaging done and was going to organise an air tightness test. Don't know if our councils have the camera, them but I'll check.

There are two bays and the roofs must be leaking air from house. I'm going to remove the tiles of these anyway as they are in poor condition, so I might be able to get in, insulate and seal it up.

The main benefit of the ewi is I'd have a total of 235mm insulation on the walls. The reps price today was 54000eu less 8000eu grant so 46000eu which includes pumping ca ity with bead, 100mm ewi, cills, pipework relocation, rendering.

As I cannot guarantee the airtightness (at moment but maybe the air tightness test will say otherwise) I couldn't commit to that expense as it may not be that beneficial.

Thanks again

At least you've found a CWI supplier! Remarkably there appears to be no suitable supplier in north London. I live on an estate of late 1990s houses all built with 50mn cavities that haven't been filled. According to the original residents, the developers claimed damp problems with the first houses and agreed with the local authority to use insulated plasterboard on the inside of the walls instead, so avoiding putting anything into the cavities. The U values of the walls as designed is around 0.4-0.48, so twice as bad as current regs. Having carefully gone through the CIGA/NIA accredited list for the south-east, I still have no offer. The best product I can find for our house is Thermabead based in Worcester, but having phoned them they do not cover our area and couldn't tell me of any contractor using their product that does. A good product could clean up on this estate, as it looks like no one has done it in the last 25 years presumably because energy costs have been too cheap. It brings into focus the problems this country will have in fighting the energy crisis, as we don't seem to have a skilled supply chain in place

The nominal U-value with plasterboard, 37mm XPS (I think the blue ones are usually XPS, but this may vary by region/manufacturer), block, aeroboard (Some DDGing suggests this may be EPS board?), beads, block, render? is 0.23. Which is reasonable, but not great. PU spray foam might get you to 0.205 which is rather better.

I'd always recommend EWI as the best way to do things, but as noted that needs the cavity filling anyway and preferably the top of the cavity sealing, so there is no downside to putting in the beads (glued ones are better so they will stay put).

That unparged dot & dab is not good - quite likely to be letting a lot of circulation happen so you won't actually get the nominal U-value, and quite likely bad airtightness overall, but pulling it all off to do it properly is inconvenient and quite possibly not practical (It'll just break). So just bunging up all the holes will probably have to do. I'd get an airtightness test (after the beads) to see where you are at. (Or mount your own fan and go looking for draughts).

So I'd say you are in a bit of a tricky zone here. Maybe good enough to get away with not EWI-ing, but it does depend how well the IWI is done, and what else is done. It would be a lot more convincing with the foam over the beads (depending what numbers your foam supplier gives - I used 0.035 and 0,027 respectively).

I'd do some PHPP sums to see where you get to with the new windows and filled cavity. Anywhere under 50kWh/m2 is acceptable (AECB retrofit standard), but closer to 25 (enerPHit) is a lot better.