The requirements from an ICAO ME-IR to a JAA ME-IR is as printed in Lasors Section E, Page 3, "Conversion Requirements".
http://www.caa.co.uk/docs/33/LASORS_06_WEB.pdf
1) The theoretical requirements (that is JAA IR(A) or JAA ATPL(A) exams passed), blah blah blah...
2) Complete a minimum of 15 hours instrument time under instruction (including 170A flight test) of which 5 hours may be in an FNPT1 or 10 hours in a FNPT2 or Flight Simulator.
5) Additional IR training considered necessary by the Head of Training of an approved FTO.
6) Pass the JAA IR(A) Skills Test.
As for schools, I'd give Bristol Flying Centre a call. I haven't attended the school yet but I will be once I have finished the ATPL exams. I have many friends passing through their doors with first time passes. However they haven't been converting their ICAO IR(A)'s so were following the fulltime course and am unsure as to how relevant this would be in regards to our situation (I'm in the same situation as you it seems).