Problems with a builder post-completion

Hi,
Having finished a major renovation using a professional contractor we now find ourselves a year down the line with the after-effects of poor work being done - including a brand new, but leaking roof. Despite the 10 year guarantee, I'm having huge difficulty getting the builder out to fix the problems. Does anyone know what is the escalation route - are there federations or ombudsmen I can turn to or do I have to go straight to the legal route?
Thanks for any advice!!!