Small claims Court. But first go through the process of getting all the information received using the FOI act. Get a hold of:
- Drawings, sing-off and client reports for your job(s)
- Ask them for number of contractors that have been and not been paid this final 2.5% over say the past 3 years
- Ask them for the amount they have had to spend to rectify jobs over the same time period.
- Any pictures showing the defects and proof of the work done to rectify them
Then if they cannot provide proof that the job was not up to the drawings/spec, or even that it was 'fixed' then tell them the final 2.5% is owed and give them 28 days to pay, after which you reserve the right to add interest and damages. If still no payment, send them a final demand giving them 7 days to pay after which you will resort to legal action. Then if still no money forthcoming go to the small claims court. It costs £10 and if/when you win, the losing side have to pay that tenner. If the judgement goes in your favour you then have the power to involve bailiffs. It's a well-trodden route and just requires you to put a bit of time to it.