Before you open anything up, have a look at the heater valve under the bonnet.
The heater valve is a black plastic valve with a couple of water hoses attached. It is down the radiator side of the block at the back, near the firewall and has a bike brake type cable going to a lever on it.
What usually happens, is the outer sheath of the cable slides in it's clamp at the valve. Full movement of the heater lever in the cabin then no longer results in full movement at the valve.
Check you get full movement of the valve to its stops when you move the heater lever. If not, unclip, reposition and reclip the cable at the valve until you do. Once you see what you have to do, it's a one minute fix.
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