Alcaidesa. Maybe you can go to one of the newspapers and tell them of your plight?
It may shame the builder into doing what he said he would originally do.
There are a lot of good journalists in Spain (Costa Blanca News) (Round Town News) (The Leader) (The Reporter) who may listen to your plight, and run a story. Or, if these papers are not in your area, contact the local press.
As long as you have all the legal signed (by the builder) documents - and your solicitor to give some quotes too - it is worth a try. And, you have nothing to lose.
Unfortunately, there appears to be too many cases of builders 'doing a runner' and getting away with it in Spain, and of course, in Britain. And in turn becomes a financial and stressful nightmare.
I have friends who had the same problem, insofar in waiting over 2 years for their house to be completed (in Mojocar).
I had a builder that had 'penalties fixed into the contract', but thankfully he was early in completion (suppose I am very lucky)
I would be wary of 'throwing more money' at the issue, via a solicitor. You state they are not 'too keen' to go ahead with matters.
Who recommended your builder?. I hope it wasn't your solicitor. As the two work 'hand in hand' in each others pockets. This scenario is not uncommon.
Hope I've been of help. Keep me posted, and I will advise, only too glad to try and get matters sorted. Keep positive.