If you are current to fly a MEP then you are qualified to fly any MEP aircraft. The training required is Familiarisation training when changing from a PA34 to a C310. It is undefined and does not require any form of sign off.
Differences training is for the specified differences and does require a sign off by an Instructor.
Another point to note is that your multi-pilot IR will not be valid on single pilot aeroplanes however, if you have a UK ATPL you will effectively have IMC priviliges. A JAA ATPL does not include the IMC privileges.