1997 Renault Megane heater running cool, engine running coo

HEATER DELIVERS SLIGHTLY WARM AIR ONLY AT HOTEST SETTINGS. ENGINE RUNS TO NORMAL TEMPERATURE STATIONARY BUT TEMP DROPS LOW WHEN DRIVEN AROUND. NEVER OVERHEATS DOES NOT LEAK OR USE WATER, HEATER CONTROLS SEEM TO WORK PROPERLY FAN ETC
TRIED BLEEDING SYSTEM WITH BLEED VAVLE UNDER BONNET, NO CHANGE. BEEN LIKE THIS FOR COUPLE OF YEARS BUT WE CAN NOW FEEL THE COLD!