If it's a major repair, then it's probably worth finding someone who is used to dealing with RIBs, rather than a general boat repair yard.
You could try Porter Brothers -- they make RIBs (their own range and for other comapnies like Tohatsu) as well as doing general GRP work:
Emsworth Yacht Harbour
Phone: 01243 377522
Fax: 01243 372929