Gav at Unit 4 in Burton will swap the engine out for you, he's as honest as the day is long and an excellent mechanic who has been working on Elises for 20 years (10 at a Lotus dealership before going it alone) and I bet he will be cheaper than anyone else. He swapped out my k series when it cried enough.
As for where to source a new or good secondhand engine I don't really know. Try posting on seloc and maybe advertising on there in the wanted classifieds? You could also try UK Lotus centre in Birmingham, they are a Lotus breakers.