That sounds like typical fuel consumption for that car, I've encountered
one running 760TD so far and it seems that's about what the owner said
he was getting for mileage. The car was an automatic though as most 700s
here are.

should be 830 +/- 50 rpm. Mine did 1:13- 1:14 if I stayed of the motorway.
First thing to check is probably injectors (min opening 140 bar, torque
70 Nm) and timing (0.9mm)
valveclearance is 0.2 and 0.4mm (cold)

Engine is also used in the VW LT, and err you've found the overdrive i hope?

I have had a 1990 740 TD manual Estate since last September.
It has just over 240,000 miles on the clock, and has 'allegedly' a
replacement engine.
The engine looks old enough to be original, however, so presumably from
another vehicle, perhaps even a van as it has VW on it.

I have had 'interesting times' with it, mainly due to lack of
engineering skill on the part of the company which sold it to me.

It seems to be running fine at the moment, apart from some problems with
oil smoke on starting from very cold, or after a long period of slow
driving (around 1,000 rpm).

The tickover is about 900 rpm, which is a little high.
I have had it given the once over by a diesel specialist and in general
the injectors etc. seem O.K.

On the subject of MPG:

when I first started driving it, I drove it much like my previous TD (a
Peugeot 405 TD) i.e. much like a petrol vehicle; rev the engine to get
more power, more than 10% over the 70mph limit on long runs.
This didn't seem to affect the MPG (40-42), not even towing made much
difference.

I was getting 29-31 mpg and wondering what was wrong.

I have now modified my driving style. I pull away slowly, change up as
soon as the revs get to 3,000 (turbo really kicks in at 2,000) and go to
overdrive as soon as possible. I try not to use more than half
throttle. Keep the revs below 3,000 on the motorway. This doesn't seem to
slow me down much - although I am slower away from lights and roundabouts,
the vehicle just keeps accelerating because of the torque and gets to
70mph almost as quickly.

I am now getting around 35mpg, which is not bad for such a large vehicle
with such an unsophisticated diesel engine.

So, keep it in the 2,000 to 3,000 rev range wherever possible.
Avoid full throttle - it doesn't seem to do much apart from throwing
unburnt fuel out the back.
Smooth and steady does it :-)

HTH
Dave R