Anyone Know Anything About Bath Deck Mounted Valves?

Got called to a job today for a good customer that I don't really want. It's a big house where I've done lots of work. The problem is they've got a posh bathroom/wet room. The bath has got what I think they call a 4 hole deck mounted set up. There's 2 valves H & C (similar to those in link), a matching diverter, a shower hose/head and the water comes out of a filler in the overflow opposite the valves, diverter, etc. The H and C valves, diverter and shower head are mounted on long side of bath on the wall side. They're not actually on the bath, but on a narrow tiled ledge between wall and bath.

The cold valve is seized through lack of use. If you try to turn the lever to turn tap on you can hear the whole valve below turning and knocking on wall. I removed the lever head and shroud from valve which exposes a circular hole in the tile filled with grout/tile adhesive and there's a chrome ring with the splined shaft sticking up out of it.

I don't have much experience with these sorts of set ups. I suppose if the valve is seized, there's nothing much I can do above the bath? Are these valves serviceable if I can get it out? What type of water connections do they normally have?

Luckily the stud wall along the side of bath has a walk in wardrobe the other side of it and owner is happy for me to cut an access hole. Don't fancy taking bath out as it's completely tiled in with large, expensive tiles.

Fecking posh bathrooms are always a 'mare.

Any help and advice appreciated.