Your old 'Group B' rating is now known as a 'Multi Engine Piston Class Rating'.
As yours has lapsed, you need to 'renew' it. The requirements differ depending upon whether it has lapsed by more than or less than 5 years. If less, then you merely need to fly a MEP Class Rating proficiency check with a Flight Examiner. If more than 5 years you need to complete appropriate theoretical and flight training at a Flight Training Organisation or Type Rating Training Organisation approved to conduct MEP training, then fly the MEP Class Rating proficiency check with a Flight Examiner.
Once your MEP Class Rating has been renewed, you need to keep it valid. To do this you must, within the last 3 months of the 12 month validity period, fly a revalidation proficiency check with a Flight Examiner AND complete 'experience' requirements. These consist of flying at least 10 'route sectors' ( take-off, not less than 15 minutes in the cruise, arrival, approach and landing) within the 12 month validity period. Once you've met these requirements, your MEP Class Rating will be revalidated for a further 12 months from the date it would have expired.
As an alternative to the 10 route sectors, you can instead fly a single route sector with a Flight Examiner as part of your MEP Class Rating revalidation proficiency check.
So, if your rating expired within the last 5 years:
Pass a MEP Class Rating renewal proficiency check
Keep your rating current with an annual MEP proficiency check in the last 3 months of the rating, plus either 10 'unassessed' route sectors or 1 route sector flown with a Flight Examiner.