That'd depend on where you wish to fly (JAA land, or FAA land). To fly a ME aircraft IFR you need to to have a ME IR, a single engine IR wouldn't enable you to fly on either of the types on your licence under IFR conditions.
I don't know if a JAA state will recognise your types and transfer them to your JAA CPL, but I believe there is an IR bridging course which is accelerated in terms of hours to complete, and as such will be cheaper.
The simple way to put it: to get yourself a job flying IFR in the UK, you need an ME IR. (Though I stand to be corrected).
Safe flying.