Well I'm no nearer to a solution than before.
First I measured the IACV, that comes back as 9.7 ohms, the plug on the loom shows one pin at 12.8v and the other appears to be earth. I was wondering why bother to have an earth wire to the IACV when it could easily be done through the chassis ground?
Anyway, I removed the ECU and stripped it to find masses of metal filings inside, some were on the legs of a rather large processor looking chip. I removed all the dust etc and rebuilt it. Still no difference, the IACV holds fully open no matter what.
Whilst I'm thinking about this, does anyone know the travel of the IACV piston? Mine only moves about 3mm from shut to fully open!