I have now solved this problem, and am posting the solution for
posterity. This is given here on the grounds that the information is
intended for legitimate use such as putting a newer engine in and old
car as I am attempting.

It seems that the injection pump does need a signal from an
immobiliser. However, it is possible to remove the electronics from
the pump, and underneath is an ordinary solenoid which can be actuated
directly as before.

You need to remove the metal cover under which the control wires
disappear on the top / end of the pump furthest from the cambelt. You
firstly need to remove the plate which mounts the back of the pump to
the engine and supports the fast idle cable sleeve. The immobliser
cover is slotted into this plate. Remove the mounting bolt from the
engine and the two allen screws and one torx screw which hold the
plate to the pump. There is however one fixing which cannot simply be
removed. There are two of these fixings which have perfectly round
heads and seem undoable. These heads are partly shielded by the
immobiliser cover which we are ultimately trying to remove. So
angle-grind the cover away from these fixings. The uppermost one
(which was completely surrounded by part of the cover) can have its
head cut off - it only holds the immobiliser cover on. The lower one
(with the hook-like surround from the cover) must be undone as it may
well hold part of the pump together, and must be replaced. It can be
undone with a hammer and chisel.  You can now pull the mounting plate
clear (you do not need to remove the fast idle cable).

There are two ordinary allen screws which can be removed from the
immobiliser cover or be cut off. Now for the tricky part. If you look
on the side of the cover facing the engine you'll see a raised part
with a hole in it. Sprung into this hole is a slotted hollow tube.
This pipe (deliberately) prevents the cover being removed as it
catches on an adjustment screw on the main pump. Try and pull the tube
out, but I failed to manage this as there is nothing to get hold of. I
removed it by angle grinding along the top of the cover along the
length of the tube (it is about 2 cm long) and prised it out that way,
after closing the tube up with a chisel to release the tension.

The the cover should slide off in the direction away from the cam
belt. There is just one wire which leads to the solenoid underneath.

It took me 4 hours in the end (and this was with the engine out).
Congrats to Bosch for making a pretty damn tamper proof system!

Thanks to Bosch and a friend at Peugeot for the tip-off that there was
a solenoid under that cover. The method of removal is my own work!

I am also pleased to say that the 1.5 engine seems to fit well in the
old car.

Jon Symonds