Dave,

(Ignore this post for now, see my other post)

Stick a MAF on it and stop muckin about, ring Seftons Auto electrical supplies in Barnsley, He could do you a good deal on MAF sensors.

ALL your symptoms suggest MAF to me incl the 0V o2 sensor reading and the low MAF output.

Is it worse from cold and gets a little better when warm? A car running as weak as this will give low vac at idle due to poor combustion efficeincy, the decell reading of 20"+ would suggest the engine is mechanically OK although there maybe more than one fault but "STICK A MAF ON IT".

Mike