Following on from the discussions we had last week about how these things work, this morning I bought one of the arm cuffs from Lloyds. The air tube is a little larger bore than that on the wrist cuff that originally came with my measurement unit, so the plug-in bit of the tube didn't immediately fit. I solved that by taking the plug-in from the wrist cuff, upping it's diameter a bit with some tape, and then inserting it into the end of the new arm cuff air tube.
When I tried the unit out, it pumped up the cuff to the correct level, as indicated by the true pressure reading on the display, and by judgement, comparing how tightly it 'strangled' my arm against how much the one at my doctor's does.
As soon as it went into the auto deflation phase, it started to pick up pulse, so I figured that it could still sense the minute variations in air pressure in the tube, caused by the vein pulsing inside the cuff. The final reading that it gave was very sensible at 130 / 85, so it would seem that as we guessed, the measurement principle is effectively 'decoupled' from whatever type of cuff is providing the input to the pressure sensor.
I will continue to use it on this basis for the next couple of weeks until my next 'genuine' assessment is due when, if it has continued to look like it's giving reasonable results, I will take it along with me, and do a direct comparison with the nurse's machine.
Arfa