This is quite a common problem on some of the older rovers (especially the metro). gunk n deposits over time slowly block up the heater matrix and reduce the flow of water through it this will reduce the heat coming out. however there maybe another reason

a few things to check first, start the car and let it idle. once its running for a while, make sure the temperature on the engine gets to its normal place then try the heaters see if there is any change in there heating. if there is no heat at all at this point its possible either the heater matrix behind the dashboard has blocked up or the regulator valve controlling the flow of warm water into the heaters has broken.

check the drivers and passengers footwells if there wet to the touch, (not slightly damp) its a strong possibility that the heater matrix has broken or is leaking and thus not warming up like normal (this will need to be fixed ASAP as it will eventually drain all your engines coolant).

hope this helps.

I had the same problem on my rover 45 2000 plate check water expansion tank i had to top up

hope this helps

Radiator leaking