Graham
You say: 'I have rotated it 360 degrees' - sorry to be pedantic, but won't that put it back to the same position? If you mean 180 degrees then that's not what is needed. With the water turned off, remove the face plate (held by 2 Allen screws) and turn the valve assembly clockwise until it stops, using the water flow knob if necessary. This should put it in the closed position. Replace the face plate, ensuring that the white plastic ring is in position and that the face plate sits flat on the body of the tap with the raised brass section of the cartridge in the cutout groove of the face plate. Tighten the Allen screws, then turn the water back on.
A few additional notes.
When replacing the seal kit, if the cartridge only had 3 seals, there is no need to use the 4th one supplied in the kit. The groove nearest the ceramic disc valve can remain unused.
All seals should be replaced in the correct orientation. Looking at the cross section of the seals, they are 'U' shaped. The top of the U should face forward on all seals with the exception of the front one which should face the wall.
If the operation of the on-off knob is stiff, the most likely cause (assuming silicone grease has been used on all seals) is possibly the ceramic disc valve itself. Remove the valve, take it apart carefully noting the sequence and lightly coat with silicone grease. Reassemble in the reverse order.
As mentioned above, silicone grease is available from all good plumbers merchants.
Part numbers:
Thermotech valve assembly: CART 06732COMPL
Ceramic Disc Valve: VLV 04070TER-OF
Seal Kit: SK 00400185
It is rare for the complete valve assembly to be required. Usually a seal kit for stiff operation or a CD valve for a failure to turn off of a previously satisfactory installation is all that is required. If water fails to turn off after fitting a seal kit, it is unlikely to need a valve - instead, just follow the notes above.
A further suggestion - consider taking photographs at all stages of disassembly. It can be a lifesaver!
Philip
August 2010