Polo 1.4 tdi Temp rising. New thermostat. Still getting hot. 64 reg

[Billy The Fish]

The temperature gauge on my wife's 64 reg Polo 1.4 tdi is going above 12 o clock. When she puts the heaters on it comes down. The fan is kicking in ok. Hose from radiator staying cold. My local mechanic (good guy) checked it and suggested thermostat as first possible issue. Stat changed but problem remains. He reckons that the next step is the water pump. I rate the guy but wanted to see if anyone else has had the issue. Apparently the stat supplier and VW said that faulty / failing stats are not uncommon. However, in our case it doesn't seem to have been the problem. All ideas would be appreciated thanks

[Billy The Fish]

Waterpump. It has a sliding sleeve that stops the waterpump from working. I assume that this is to get the car hotter quicker. Except that it requires the sleeve to move backwards and forwards regularly. And there is the problem.