Discovery 2 heater issue

The heater in the truck is barely getting warm......on form it is roasting. We have the climate control, dual. Each side goes cold on low but on hot only just get slightly warm air to blow.
The truck gets to temperature so it is not running cold....middle of gauge.
I know I have had this issue before after draining coolant with airlocks but it has not been drained.
I have used the bleed on top of the rad hose.
Any ideas received greatfully!

Most probably the heater core is clogged, happened to many... the best way to achieve a perfect result is to replace it as it's full of "stubborn" deposits after 20+ years, doable in situ with glovebox removed and some contorsionism and mechanical skills but if you are not up to that you can try the improvisation which would improve heating a bit: ... put a bottle of THIS in the reservoir and drive it at least 10 miles or let it idle for at least half an hour while you push the throttle several times and keep it above 2000rpm then drain and refill and put another bottle in the fresh coolant mix and that's it, ... if after few days you see the new coolant too dark drain and refill again... if no joy heater core replacement is necessary.