The coolant was full of gunk, which I can only presume was something similar to radweld. I dropped the coolant, did the work on the car, started it up and the engine was mixing oil/water. I can only presume that the chap who sold me the car knew about it and threw in the headstop/radweld to get shot of it. The plugs were loose in the engine too. Bit of a mess.
Anyway - I seem to have isolated the problem. The adaptor plate was leaking air through cyl 5., causing high idle and the stupid sound (I hope) The inlet port on number 5 was full of hylomar sealant, where as all others were clean. I'm in the process of cleaning everything up, then i'll reassemble.
Fingers crossed and thanks for your help.
Chris