Within the power limits dictated by the existing cabling, breakers etc.
It would be poor advice to encourage the OP to wang in a 10.8kW unit on existing 6mm² cable, for example, without qualified on-site assessment.
Refurb kits for Ideal fill valves are available -diaphragm washer ,billet etc.
CBA to research the part number just now but saves sweat if access is poor e.g. back to wall wc
Snickers toolcoat always did me back in the day when I wore separate trousers (my winter workwear is snickers onepiece overalls with the holster pockets).
I'm unlikely to change my brand loyalty now.
A thin bead of transparent silicone applied to the top and side (cleaned!) surfaces.
Or: demount mixer, remove cover plates and gob up the gap between pipe and tile.
May only require https://www.screwfix.com/p/arctic-hayes-siamp-diaphragm-washer-29mm/5006j
Accessed by undoing the large knurled nut on the fill valve. water off, obvs.
Going by the avocado tint and the presence of 4(!) fixing bolts I'd date that to the very early seventies.
Not saying it can't be fixed but IMO retirement would be the kinder option.
McAlpine products are generally excellent, better engineered and more solid than cheaper alternatives. Never saw a bad one.
Can't imagine what went wrong in this case; dare I suggest user error?