FMC,
You will need a standard LPC/OPC check with a tick on it as an ATPL skills test which will require a manual ILS to be flown. That's the only difference.
You will then need to present your log books to the CAA with ALL pic u/s hours countersigned by the appropriate Captains or a letter from your employer stating that the hours accumulated as PIC U/S are correct.
Then pay £216 for the privilige of getting a crappy green folder
You don't need to be a Captain as that would then mess the system up for FO's with a CPL trying to get an ATPL on the same aircraft type. Can't be a Captain without an ATPL, can't get an ATPL without being a Captain..... bugger!
Enjoy