After we had received the letter from the solicitors, Him Outdoors remembered that he had been given some free insurance from our expensive french insurer that might include something to do with legal matters.
He went to see the Axa man and sure enough (something right in this saga!) although the amount isn't enough to be covered completely, we do have insurance that enables us to ask their legal team what we should do. The Axa man also offered to come and see the stairs to see if we were being sensible or not and to write a letter to the stairman/solicitor.
After he had visited the general concensus was that he would write a letter taking the attitude that we were not going to pay the bill yet because the stairs were not finished. List the things that could be mended in situ, mention the things that were wrong but presumabley unsolvable in situ - like it goes in front of the door underneath it, and it's put in crooked. And see what happens.
This has now been done and sent off with 'avis de reception' and we await the next stage.
From the happy corner the Axa man has been lovely, very helpful and prompt.