Thanks everyone. Was cut off in mid sentence as I suddenly felt ill and have been asleep. Anyway, thanks for the patience and help. I have learnt quite a lot. Mainly that I know less about domestic wiring issues than I thought! The guidance here has been very generous.
I am in the process of doing up the whole house, which is an old barn conversion, and replacing sockets as I have gone along. All new circuits have been either installed by a pro (the one currently in Eire) or by me and then checked by him.
I was unfortunate as said above that just as I was replacing sockets in a room (they all needed to be moved outwards due to panelling being installed) and so old brass sockets were switched to new SS Hagers, the fault occurred. At first it was a slow fault - in which I could reset the RCD and then it would trip after a few minutes. Then after a while would not reset at all. I just assumed for a long time that I had created the problem, even though I have replaced hundred of sockets in my time and am very meticulous.
The pump is a Draper unit and must be at least 10 years old. I fitted the outside socket a year ago, replacing an old one that was there before, and it was a quality unit and tested. The submersible pump is crucial and I have a spare at hand and will install that tomorrow.
The fault has done me a favour as it has made me trace the circuit round the house, and I now know it goes from the socket where I had been working, up through plastic conduit to a large bedroom where it feeds one single and two double sockets, branches off as a weird spur to a dressing room with two double sockets, then back down, through a hall and back to the CU. The bedroom is not yet refurbished and in the tracing process I have also discovered one loose socket that some dork has put a radiator across. You can't make it up. Next job will be stripping that lot out and replacing the wiring circuit entirely.
I have also made the decision to replace the 2018 CU with an entirely new Hager RCBO unit. The circuits are just confusing and illogical, and I would rather have the security of RCBOs, anti surge etc. My sparks chose Hager oriiginally as he said they are good quality sockets and CUs. The CUs elsewhere that he fitted (to new build parts and three outbuildings) are all Hager RCBOs.
This forum is a very knowledgeable resource. Super helpful.
If you are wondering why a DIYer is doing electrical stuff, it is simply that I am doing the refurb myself in my spare time, which includes a lot of new build, so it can take me ages to get some jobs done. The electrical work is therefore very piecemeal, so I found a friendly electrician who will guide and advise me, then check my work and do any Part Ps before we sign off on each section. That's why I very recently bought the Megger test unit etc, so I can do the tests he will do, make sure my work is OK before he checks it with his rig. He was going to guide me but it was Christmas /NY and so he is away with his family on holiday. Sods law.