Jordan, ask Falkland exactly what work they plan to do for their quote. Drop the details back in here before committing. They have a wide range of experience, and have looked after various members' Elises over the years, and it would be unfair to write off their expertise without a little more exploration first.
However, as suggested above, £600 is only going to buy a few hours' labour and since a lot of that can easily be consumed simply in removing and refitting the head - without doing very much else other than dropping a new gasket in - you do need to be careful. It may well get it running again for a few thousand miles but the first time the engine is put under sustained load or extreme heat cycles, and you might get let down again
Several of the guys who've responded above *really* know their stuff, but none is running a formal service so unfortunately you can't just book it in with them

(more's the pity...)
You should also contact Craig Moncrieff at Murray Lotus and ask the same question re: procedures / investigations. And a further thought is Dreadnought Garage in Callander, who have stacks of K-series experience. Obviously costs will vary between them all but it's the *content* of the work they'd each do which matters for a long-lived solution.
Campbell