Hello again.
Sorry for the delay getting back but here's the reason.
To be completely honest with you, what prompted my original post was getting a quote from a local firm of electricians for some minor kitchen work (£300). That seemed pretty steep to me even before the guy looked at my consumer unit and told me that not only could he not do the kitchen work before the CU was changed for an RCD type, he could, and should, lock down my supply until that work was done (£250). Not good news; I've just blown some record, never-to-be-repeated overtime on a new kitchen and £550 is money I just don't have.
Anyway, by coincidence, I just got a letter from the local Southern Electric office. They're offering a free visual wiring inspection to owner-occupied properties over 15 years old so I went for it. Their inspector has just been and I thought you might be interested in what he had to say (particularly JohnD and plugwash to whom many thanks).
Basically he found the wiring all sound, the (PME) earthing system in good order and with recently uprated earth cabling, earth bonding all good throughout the place and the ONLY recommendation he came up with was to put an RCBO on the socket ring. Furthermore he said that Southern electric prefer individual RCBOs to split boards or whole boards that are RCD-protected (although SE would fit those if requested).
At this point I told him I'd been advised to get an RCD-protected unit but didn't go for it on the grounds of cost and also because I don't like the idea of my whole place being plunged into darkness if there's a fault on one circuit (I mean, that seems to undermine one of the main reasons for having separate circuits in the first place).Anyway he agreed with that, told me the CU is a current model and then wrote down the details of the RCBO I need (£30ish not £250). He left saying my wiring was in good condition and, apart from the lack of a RCBO on the socket ring, it complies with current standards and there was nothing else of concern.
So a big "thank you" to you all, especially JohnD