Sorry, I was not being very clear: the barn is over 150 years old. It was converted into a home 30 years ago.

Try getting it done -- they will do a survey with a probe camera if they are good. You could sign a disclaimer and you could also get a grant. Use polystyrene beads not fibres.

I just had a first visit from a CWI company called Viscount. Their rep said that because the outer stone skin is 600mm thick it will require a visit from one of their specialists with a long drill and camera. The rep said their main concern is porosity of stone walls and the possibility that CWI will induce damp problems even though the fill itself (beads) is waterproof. Rep said that if they decide they can do the job then we will get a grant.

good news that -- you could let them drill from inside
 
is the stone pointed and built with lime mortar?
 
any leaking gutters or high ground levels?

Yes - I don't mind where they drill. Can patch any damage up. I will remember to mention when the specialist visits.
 
Yes the stone is pointed, mostly cement mortar I think. Some pointing is in poor condition, some is absent. The stone walls were built with lime mortar. Repointing is in my plans for future.
 
No leaking gutters as I just had them replaced. One corner of the house has high ground level.

Just had a second visit from Viscount, but I did not get the impression that this rep was any sort of specialist. The first thing he said was that they don't do stone walls "because they are uneven on the inside". I showed him the gap where the inside of the outer stone wall is visible and clearly flat and then he said "We don't have a drill long enough to go through that". He showed me his longest drill which was 300mm - indeed, too short. So I asked him about drilling from the inside of the house through the 150mm blockwork. He said "Health and Safety won't let us drill from the inside in case we hit electrical cables. They only let us do that on new build." He also said that they (Viscount) use mineral wool not beads, and that the only firm he knows are using beads is Miller Pattison. I plan to contact them today but does anyone on this forum know of a firm specialising in old buildings of unusual construction?

That your man walked away is a sign of honesty you hardly find nowadays. Getting a 1m long drill from the tool hire shop wouldn't be a problem....
Your wall has - propably unvisible to the untrained eye - more problems than a high thermal conductivity.
Contact a real building/construction engineer. He must check what can be done, make suggestions, figure out costs and workability.

Had 3 visits from Miller-Pattison. They drilled holes from the inside, through the 100mm blockwork, at 4 places in the house. They put a Borescope in and the cavity looks suitable for insulation using EPS beads - "Eco Beads" - they say. They have done a detailed little survey. Say their guarantee is 25 years and if they go bust CIGA step in. I'm inclined to get it done even though there aren't grants available for this kind of CWI so I'll be paying the full price. Before I sign, does anyone think there is a good reason NOT to do this?

Wonderful news!!!
 
do I get commission?

tony: you get a beer if you are ever in the vicinity :)
 
You don't think there are any downsides to getting it done then?