I would suggest checking the radiator cap (when cold) for gunk around the rim and rubber. This would give you an idea if the system needs a clean and flush.
If so, that is a fairly straight forward job by removing the bottom hose and refilling the system and rechecking the fluid level while the motor is running. Remember the anti/corrosion/freeze additive.
Normally my 03 RS heats up to 52C (just off LO) in about 3 minutes and up to 83C in about 5 minutes.
tell us how you go..
If unsure ask your dealer..