I don't have and never have had any such problems, but I seem to remember it was suggested that there was only one cabin temperature sensor (I expected one for each side). That sensor is down to the left of the steering column. the small vent like aperture. Apparently it uses a small computer type fan to draw air into it.

The cold passenger side is most likely (from what I have read) due to the matrix being partially blocked. The design of the matrix means water does not have to flow to all parts of it and as it gradually blocks it does so from the passenger side first, which is furthest from the in/out pipes.

Take it out and give it a good flush and clean out.

Another possibility could suggest, is the air mixer flaps stuck in the cold air position.

Water in the foot well might or might not be connected. That could be due to a matrix leak with a possible air lock in the matrix causing your problem (does the rad needs to be topped up frequently?). Water overflowing from a flooded plenum, or water overflowing from a blocked sun-roof drain (if you have one).

Thanks everybody, all sorted now, the water in the footwell was the sunroof drain not sealed well where it went through the bulk head and partially blocked........and the heater was as highlighted above the mixer flap modulator was disconnected (hence why the mechanic said it was disconnected and worked, but called it a sensor???) I have now replaced this and all is well....All this being sorted but today found i had no reversing lights I looked through the fuses and lamps still nothing then turned to the 'Rave' (Legit) ish disc and found the gearbox switch sensor physically not connected, this must have been left off for the whole year since the clutch was replaced now its off for the MOT tomorrow so lets see what other surprises it has instore for me.