Legal letter writer required

I instructed a lawyer in 2009 to mount an action against Insurers who repudiated my Professional Indemnity claim in 2003. The lawyer (sole practitioner) then joined a large firm of solicitors as a partner. He was still dealing with my affairs whilst in his new position. His workload was such that he was unaware that subsequent to issuing legal action in the High Court against Insurers, that they responded and a Hearing date was set. I was never informed of a) the contents of the letter from Insurers to my solicitor b) Date of Hearing. It tarnspired that the firm of solicitors sent a represntative to High Court on my behalf, without my knowledge andwe lost the claim. I was ordered to pay £30,000 costs which was reduced to £10,000. I now wish to present a professional indemnity claim against this firm of solicitors for negligence and require a letter to be formulated to be the basis of the claim as well as formal complaint to the Law Society