No Central Heating - Promax 24 HE?

Hello. I have a similar system to that mentioned above. Since the weather turned colder a couple of weeks ago we have been unable to have heating without the hot water being on. Once the hot water reaches the desired temperature, the boiler shuts off and the only way to get heating again is to use the hot water up. I already had a spare programmer originally changed for visual reasons which I swapped back but no change. I have rung approximately 15 local plumbers and all seem too busy to come out! Full spec is as follows:

Potterton Promax 24 HE
Heatae Sadia Megaflow

Pictures attached!

Any pointers would be appreciated.