Proline Fridge Freezer TFP135B

Hi all this fridge freezer is now 6 years. Problems started a week ago.
First of all can I just mention that the cycle of my F/Freezer was on for approx 30 mins and off for about an hour. This has worked fine and believe this is how it should work – on and off. The dial thermostate on this model is 0-7 and I had the dial on 5.

Ok so a week back, been out shopping put in food – I had noticed I accidentially left the door open. The door must have been open for 15 mins max – although ice drops built up on back wall of fridge. Closed door thought nothing of it. After about 30 mins the f/freezer shut off – again nothing unusual. That evening I noticed that the f/freezer then started to stay on for say 2/3 hours before turning it self off but then when it did eventually turn itself off the f/freezer wouldn’t switch on into life for another 2/3 hours – by that time the freezer section is close to 0oC. My model of fridge is a flat back so I cannot see any of the coil (to try to clean them/hoover). I switched the f/freezer off – totally for a week. Yesterday put it back on – it kicked into life start away as temp 1 and turned up to 7 to get the freezer at the right temperature. Obviously still have an issue as although the f/freezer is at the right temperature, it f/freezer doesn’t want to turn itself off (someimes it does auto turn off but only after 2/3 hours) and the same when coming back on. There is obviously some life in the termostate as it manually lets me turn the dial on and off but it doesn’t want to do it automatically like it use to (ie on for 30 mins and then off for about an hour and so on).

Have the f/freezer basically had it. I’ve tried searching for a replacement thermostate in case its that but dont think there is one out there. i know nothing about electrics and wont even start taking backs of things or anything. Just done understand why it was perfectly fine and now 🙁 it doesn’t.

Thank you in advance for any help you can offer me.