I can't really help you I'm afraid. I converted an ATPL to ATPL and it's slightly different.
I believe Tayside up in Scotland have done a few of these conversions.
You'll be up for an IR (min of 15 hours, some can be done in a sim) and a CPL check (I think).
I think you need to budget around £300/hr for a twin.
Remember to add on the CAA's charges. I think they're around £600 at the moment.
You'll need to budget for an MCC. Possibly £2,000.