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!).
Thanks again
Adrian

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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)