I have been busy but not very effective.
I have done the following
1. Removed the 4 inlet manifold tubes (including the injectors)
2. Replaced the 3 "old" injectors with 3 new injectors and 4 new o-rings. All the injectors are now the same.
3. Replaced the inlet manifold gaskets
4. Re-installed all of the above
Car started and ran better (maybe) but now it is definitely running on only 3 cylinders.... number 4 is not working. (Removing either Number 4 spark lead or injector plug produces no change in RPM. On the remaining 3, now a very definite drop in RPM)
There is spark at the distributor for number 4 cylinder.
I don't know of any easy way of checking whether number 4 injector is getting power when it should (multimeter would not measure anything I assume). But as the injectors operate in pairs, it is unlikely that there is a fault in ECU except perhaps in the cable run to the injectors.
So I assume that the problem is
1. Faulty Number 4 spark plug (brand new that they all are, I recently installed 8 new plugs on my MG and one was faulty)
2. Faulty Number 4 spark plug cable
3. No power to fuel injector 4 (I assume it is not the injector both because it is brand new and also because number 4 was a problem before I replaced the old injector). Probably faulty connector or break in cable
4. Although compression check was fine, perhaps there is a fault in piston/cylinder somewhere (I hope not!) "Daffyduck", I noted your comment about oil in the cylinder but where would oil come from? I did take the oil cap off but it made no difference
So some more work ... plug another (old) injector into cold start line and number 4 cable and see what happens. Try new spark plug, depending on what I find, chase the cables again back to ECU. I could replace the loom to the injectors if needed but I will need to buy some more Molex connectors (do all DS's use these?)