Chris, thank you for your in-depth replies. Greatly appreciated, as are all inputs here, guys.
I have just clambered into the loft and remeasured the tank. It is 76cm to the bottom of the tank from the plasterboard. Adding the distance from the ceiling in the bathroom below to the top of the showerhead is a metre total. So a bare minimum of 0.1 bar. Not great.
I cannot raise the tank at all. Not without removing roof tiles and timber....!
Shorky: Thanks for your comments. The cold feed comes direct off the side of the tank. The hot comes from the pipe rising into the loft but not direct from the top of the cyl., no.
I post below two pix of the pipework feeding the Triton Tesla thermostatic Mixer Valve:
First is the cold leaving near the bottom of the cold tank.
Second, the hot, tapped off the hot feed off the cyl. as it comes up through the ceiling into the loft.
Shorky, if I understand your post correctly, are you saying the 15mm stumps should be done away with? If so, I tried to persuade the plumber to do this by fitting the 22mm - 1/2" tap connectors someone (bless you, I've forgotten who!) recommended a few pages back and he gave me several bad-tempered reasons why he could not fit them. "Anyway, " he said, "It is 15mm inside the valve, so what is the point of supplying it with 22mm?"
I really seem stuck as to what to do next.
We want to stall the plumber again on the shower and get him to finish everything else. He can spend no more time after tomorrow so that will be that.
My options for rectifying the shower work at the moment seem to be either one of these or a combination, possibly of all three?:
1. Remove obstructive NRV and limiter from Tesla.
2. Re-configure the pipework for a more direct feed from the cylinder.
3. Replace the Tesla with an Aqualisa, "if it'll work on gravity, they mean it".
4. Add a pump.
Chris: I've just read my Tesla manual. There are 2 NRV's, one on each inlet. There's no talk of a flow limiter, but there is mention of a flow regulator. This, it says, is only for use on high pressure systems.