The cost analysis for banger repairs is pretty straightforward.
Does the repair cost more than it would add in value...
Let's assume that the car is worth £800 in a safe, usable condition.
Let's assume that the following issues are present and the work is being done by you, so no labour.
1. The aircon doesn't work. Value affected by £100. Repair, new condenser plus gas, so £110. If aircon is important, you might swallow the £10. If not, leave it.
2. The waterpump has failed. Value affected by £600. Repair, water pump cambelt coolant, so £200. Fix it.
3. Cambelt snaps. Value affected by £600. Repair, new engine plus service items and fluids, say £1,000. Scrap it and save £800.
Anything not affecting the safety or usability of the car can be largely ignored.
Servicing, tyres etc should be predictable enough to budget for. For example scrapping a car that only needs four tyres is stupid, as the replacement car will likely need things addressing plus the purchase price. (obviously if you're looking to sell your car, then obviously best to maximise the value of it... but, bear in mind that spending £300 on it just to gain £200 in sale value is a waste of £100.
If you want to keep a worthless car, then you need a money no object approach to repairs, provided your household budget can accommodate it. If the budget cannot tolerate it, then cut your losses and buy something less worn out. This isn't the same thing as newer/more expensive.