Have you tried a new thermostat does the engine temp get to norm?

The valve is a easy fit also you may have air in the heater it's easy to bleed & if you have to remove the heater unit thats easy too.

Look in the Haynes manual it's all there what to do.

im having an un usual pro in that respect. but i would sugest before you purchase a new hcv that you test the old one first.

its easy to replace even for a novice but first check weather the pipes comming out of the hcv are hot or cold when engine at normal temp or just hot up to the heater control valve.

set your heat dial to just over half way and then put your hand on the silver part of the hcv and you should feel it openning and closing at around every second or 2. if this is the case your hcv should be fine. if not change it.

second stage, remove the stereo and there are 2 screws holding in the heater pannel, remove the 2 screws and pull out the unit. again very easy. behind the temp dial you will see a black plastic cover the size of a credit card. undo the 2 screws and you will find a curcit board, a blister on one of the diods will tell you if that has gone.

also try removing the plug from the heater control valve and driving with blower on. with the plug removed it should be hot , if not then its not the hcv.

if both these dont work. then perhaps you have a curculation problem. let me know how you get on as i coul;d keep going.