Many thanks Gilly & Mick
Tested it in the garage today - you were exactly right in all that you said Gilly. It certainly appears to be working. It operates at a no-load engine speed of about 1900 rpm - there is a slight change in engine note to accompany it.
I understand it cuts out again at a higher rpm but it didn't seem to up to about 4000 rpm (didn't want to rev it much higher off-load!). I'm sure it does cut out as it pulls like a train at high revs, its just at lower revs that it seems to lack the power I would expect.
As regards the oil pressure fluctuation I shall watch the gauge with interest in future, though I would be surpised if I saw anything - the oil pressure is always on maximum at anything over 800 rpm (despite its 197000 miles!).
90 300TE 24v (M104 eng) 207K miles
87 300TE (M103 eng) 269K miles (and never had the head off!)
86 300TE (M103 eng) 230K+ miles (donor car)
95 Toyota Lucida diesel 4wd
86 200T 165K miles (sold)