Glowworm micron 40FF Thermistor Rod?

A month ago I was having to reset the boiler frequently because the Reset light would come on. Eventually it stopped working. Using the flashing light sequence I worked out that I had to replace the thermistor.
So I ordered one (THERMISTOR CONTROL 30-50FF - 2000802635) for £15 from boilersparesdirect. When it arrived I noticed that the original thermistor rod was about 10cm long and the new one is about 3cm long.
It fitted OK with the existing PCB so I though it would be fine and the boiler indeed started working again.

However after few hours I realised that the central heating would only work when the the boiler fires for hot water. So to get the central heating working I'm having to run the hot water tap and eventually the heaters would warm-up.

May be it is to do with the length of the rod or may be the connector to the PCB is different. I got the part number from the manual. However when I try look for it all part shops suggest that the original part has now been replaced with the one I bought. I'm not able to find the original Thermistor.
Anyone able to help?
Thanks.