Hi, my rear tyres will need replacing soon.. and not being technically minded.. I have no idea how TPMS works..

Does the garage swap over the sensors?..or are new sensors required?.. 

I just tried to see how much my tyres would be fitted from Blackcircles via a local garage and there was an 'add TPMS valve replacement' at £20 a tyre??

TPMS uses the abs sensors to measure the rolling radius of the tyre. You don't need the TPMS valve.

Just swap the tyres and reset the TPMS system (simply press the TPMS button and hold it for a few seconds). Basically you can skip that but it's highly possible that you will see a warning soon. Then you can step out of the car, see if nothing's wrong (probably everything will be fine) and reset the system then

But make sure they are the right pressure before resetting or else any differences will be built into the baseline reading...

I know the tyre garage should do this but they don't always and if you are like me you may prefer to inflate to 34 psi all round anyway.