Well it's sorted, just before Christmas I bought a new RWC mixing valve.
So it was the valve that was faulty but not in the way I had thought.
Turns out the brass tap mechanism that was fitted to the original unit is incompatible with the main casting and thermostat!
Basically the new tap has a different rear face that essentially makes it shallower and allows the water to flow and the stat to open. I couldn't resist so took the new one apart to investigate!
In the images attached you can see the new assembled unit at room temp with the stat very slightly open from the tap face, i.e. allowing a small flow of returned cooled water from under the floor. The old unit is jammed shut at any temp.
In fact as I'd discovered before, the old unit doesn't allow the stat to move at all, and worse than that as the tap face extends as you dial the temp down it crushes the stat pin into the wax and damages the stat!
The interesting thing is that I've carefully measured all of the new casting and it is identical to the old one so it's only the new tap that is different.
In the new year I'll ask RWC what is going on, because I'm interested to know what has happened here as the valves have the same part no??
The retailer was pretty poor, basically they only supply a whole kit with this valve and pump, so I'd have to pay packaging to return, £250 for a new valve and inferior pump or over 500 for a new kit and then wait weeks for a warranty claim. In the end (and actually suggested by the retailer - cheek) I bought a new valve for £100 from a different supplier to get it in a couple of days and took the cost myself. Bit annoyed and won't be recommending them to anyone soon.
Bottom line is there was nothing wrong with my self-install, it was a really odd, hard to diagnose fault with the mixing valve so I thought I'd share the detail.
I now have a steady 45 degree flow and a very happy cat lying on the new floor
New unit slightly open to return flow at room temp:
New tap has the protruding ring, but shallower main base surround
At initial low temp the input thermometer reads what I'd expect now, at about 42 on the knob it runs at a steady 45 regardless of other demand on the boiler