To recap - number 4 injector packed up, and it took me a while and some head scratching to diagnose, then replace it.
Then immediately after the car produced various codes relating to number 1 injector, which was puzzling to say the least.
It would run but only on 3 pots.
Lots of trying checking various things, in between spending much time at work and waiting for some dry weather meant it dragged on for weeks.
Bought a Bluetooth diagnostics thingy last week, which gave the same fault codes and lots of live data I didn't understand.
Went back to basics today, and had a think. I had swapped injectors from pot 1 to pot 4 but the fault codes stayed the same so the problem wasn't the injector but somewhere before it.
Then with the engine running, pulled the connector from pot 2 and measured the voltage, which was just over 2 volts. Did the same on pot 1 and got the same result.
Logic told me the only thing left was a bad connection between the plug and injector. Carefully bent the little contacts on the plug inwards slightly and hey presto. All is well again.
Dave the builder told me to check connections at the plugs quite a while ago. Should have took it more seriously than I did.
Thanks for all the advice and encouragement, and even the solution in the end Dave.
Your a star.