Hi @kezo I haven’t wanted to post before now for fear of tempting fate
Anyway, update as follows. The new supply to the extension went live last Thursday. It ended up taking him just over 2 days to do it (thankfully his hourly rate is very reasonable). I am pleased and relieved to report that we haven’t had any power trips since. The external thick black cable on a rendered wall looks awful, but I need to get the house painted next year (we have it done every 10 years) so painting the cable will be part of it
Anyway, update as follows. The new supply to the extension went live last Thursday. It ended up taking him just over 2 days to do it (thankfully his hourly rate is very reasonable). I am pleased and relieved to report that we haven’t had any power trips since. The external thick black cable on a rendered wall looks awful, but I need to get the house painted next year (we have it done every 10 years) so painting the cable will be part of it 
We aren’t 100% confident that the problem is resolved yet as we haven’t had any rain since and some of the previous trips had been during or after rainfall, but there is rain due from Wednesday onwards so it will have a test before we go on holiday. Then, of course, it will be interesting to see if the power holds during the 2 weeks we are away. Once again, sincere thanks for all your advice and assistance. My stress levels have now eased!I honestly think you can put your mind a ease and just go away and enjoy yourself 🙂
Going forward, the Type C MCB in the meter box replacement consumer will protect the new supply cable as should and help with any surge’s coming from the transformer, leaving the MCB’s in the extension unit to do their job protecting the individul. circuits they supply.