I have three questions.
He still has the original CU. Later (but maybe not much later) a 6 mm^2 circuit has been added for a shower heater (probably a predecessor of the present one). The layout of the present board is as follows.
Incomer -> Supply Company Fuse -> New Smart Meter -> Old Wylex current operated ELCB (100 ma trip) -> Henley Block.
From this the supply splits to the original CU which has plug-in MCBs, and what I can only describe as a one-way CU for the shower, this has a switch (havn't checked if it is one or two pole) with an integral 30A plug-in MCB. This suggests to me that the original shower had a lower rating than 8.5kw, I'd have expected to see a 40A trip to protect an 8.5kW heater circuit but maybe you can get away with 30A for short showers.
From there is a simple short run of 6 mm^2 t&e. It's surface mounted on the "board", disappears into the ceiling, and runs up about two metres of wall to the shower unit. It runs behind ceramic tiles on an outside wall which (I guess from the age) might be normal rather than lightweight concrete blocks for the inner skin. The outer skin is a single layer of brickwork. Exterior walls are plastered inside, not plasterboard.
So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).
Clipped direct is OK to 47A. What does the team think about the wire run behind the tiles? I'm inclined to think that while it *might* run slightly hotter than clipped direct, the insulation is not particularly good and it can't set fire to anything sandwiched between blockwork and tiles (presumably chased into the original plaster). Do you think he could go up to 9.5kW?
Second question, the overall RCD protection with 100mA trip is a bit out of date, should he get this changed to 30mA? I know, in an ideal world he'd swap the lot for a metal split CU compliant with 17th edition, but he's also renovating his original flat prior to sale, so he's not rolling in cash. The rest of the electrics doesn't look too bad and doesn't appear to have been hacked about significantly.
Third question, the overcurrent protection for the shower circuit is this unit with a combined MCB and switch. I guess he needs to go up to 40A especially if he changes to 9.5kW. But does he need a switch here? After all, he can isolate the shower circuit by tripping the RCD device (at the price of turning off the rest of the house). He has, of course, the normal double pole ceiling mounted pull switch outside the shower cubicle. If he does need a switch, can anyone suggest a reasonably compact switch and overcurrent protection unit? I guess it doesn't have to have an MCB, could he use an industrial type 40A cartridge fuse?