Have you looked at the figures over a longer period?
At the end of the 4 years, lease vs buy isn't going to put you in the same position - to be a fair comparison, the "sell current car" cell in the last column should be -£166.67 (£8k/4 years)
Your current risky car is almost 20 years old, the car you'll have after 4 years will be 6-7 years old, so it's not like you'll need to replace it straight away - it's still going to be perfectly useable, and massively more reliable than the current one
I realise it sounds like I'm really trying to push the "buy a used one" option, it just seems you're looking at the total cost over the 4 years without taking into account what comes after, apologies if that's not the case.