If you revalidate your IR as an SEP IR you will find yourself at a disadvantage as a SEP IR holder is not able to fly an MEP at a later date IFR, however a MEP IR holder can fly SEP IFR any time. In order to do a type rating you will require a MEIR, however not a valid MEP.
Your MEP needs to be revalidated once every three years or you will have to do the MEP course again. There is a little grey area hear however not an expert on it as the rules have recently changed, as you have stated already. The main reason most people keep this current is to keep the option of light aircraft MEP jobs open if they come along.
The most cost effective way to sort this out is do an MEP revalidation each time you revalidate your IR in the aircraft (every other year) as this will save on multi hire rates and its only one examiner fee. You will have to do this with a renewals examiner though, although not directly with the CAA.
Give your local school a ring and shop around! the prices tend to vary quite a bit depending on where you are.