VW Polo 1.4 Petrol Overheating when idle and blows COLD air inside heaters?

VW Polo 1.4 petrol, Automatic, 1998 S-reg, 42,000m. Car has just had thermostat replaced, as was overheating before and still it's Blowing COLD air and NOT Hot air from the internal fan switched on vents, when air blower switched on in any type of position or speed.
Cannot see any blockages, as water pipes were taken off. Water is under pressure and pipes get hot. The car fan does come on okay at 90C, and car overheats when in idle state to temp at 3/4 level, when the fan kicks in.
But whilst driving, it polo does NOT overheat at all as temperature needle stays constant at 90C in the middle position of temp gauge.
Need a solution as to why polo blows COLD air internally. Have been asked to replace the Water Pump and the cam belt for next stage and if this fails, then the dreaded heater matrix by replacing behind the dash board - major job?

Can anyone suggest or help with their thoughts please?