Right back on topic. Luckily I have the print off which my VW tech mate gave me when a number of error codes were logged. I had all of them so the PCV valve was changed as a matter of course. This is from 'Elsapro technical service handbook'
Technical product infomation
Erratic idle speed, entry 08583, 04759 or 08825.
Customer statement/ workshop findings
Erratic idle speed. The following entries are logged in the engine control unit.
08583 P2187 bank 1, fuel measuring system too lean at idle speed
04759 P1297 turbocarger- throttle valve connection, pressure loss
08825 P2279 air leaks in intake system
Technical background
Leaking non-return valve in the pressure control valve.
There's also a picture of the PCV which is circled as part of an exploded diagram.
Any use?

Like I say this didn't fix my irradic idle, but there was a fair bit of oil in the DV (which was then replaced and fixed the problem, multiple rip's in the rubber diapragm) which apparently is an indication of a knackered PCV valve. My car is an early 54 plate and the PCV I took off had a 2004 date on, so it was replaced with a modified version that VW strted to use later on in production due to the failure rate of the originals like I had.
Sam