3.6kW @ 230V is 15.6A
You will be fine with a 4mm T&E cable running back to a 20A RCD
The size of the cable depends on the length of the run and what is is encased in (plaster, etc)
When investigating further I find I have a unused 6mm T&E cable running back to a 32A Contactum MCB, would this be OK to use? Should I replace the 32A MCB with a 20A MCB (these come in 3 types, which one do I use) ?
The cable will be fine, you will need to down-size the breaker with a 20A MCB - get the make and model from the 32A one that is currently installed, or take it out and take it to a electrical distributor and get them to match it.