Once the engine is warm, the heater only becomes hot when you rev the engine, at idle top pipe going into core is warm, bottom luke warm -air comiong into car is cold.

I have flushed the heater core, (attaching hose pipe to exit hose), but still problem remains.

I suspect some 'Carlube' radiator sealer, when I poured it in the bottom of the bottle had all congealed, maybe some of that has blocked something.

I assume the heater core vents any trapped air by itself ?