UK lawyer to get architect to value roof extension contract variations correctly

The project is a roof extension in London using a modified JCT Design and Build Contract. The architect I employed, who is acting as Employer's Agent, has taken a cavalier attitude to valuation of variations in his draft final account and the final certificate is due to be issued early April. The archiect has failed to respond to my emails (i)detailing matters he has failed to address in his valuations to date and (ii)holding him responsible for costs incurred under the contract due to his failure to pass on to the contractor instructions I had given him. The costs involved are relatively small (less than £10K)but I cannot accept that my architect's response is to refuse to communicate. So in the first instance I am looking for a 'shot across the bows' letter so he understands that he must address these issues. If that doesn't work I will presumably need to pursue action both in the courts and with the Architects Registration Board, and these consequences need to be sufficiently clear in the letter without being unduly aggressive.

I appreciate that this is not the most enticing of assignments, but I am just about to set up a series of new businesses and would much prefer to work direct with a freelance than via the normal legal firms... so follow on work is probable. I already have another matter which I can outline if I get a satisfactory response top this one.