If you have a car for many years, it is worthy doing the repairs because you know the state of the car. If you buy another car, you will be forced to do the first repair and when the next one arrives, you will do it again, etc.
You can do the maths, how much you have paid for the car and how much for repairs and then you will have a monthly cost for each of them. You should add the quote for the new repairs in the sum and check how it divides over the months you had the car.
If you had the car for 10 years, any big repairs will be very "cheap" over these years.
A nearly new 6 will cost more than £300 per month on finance and you will be trapped to it for many years. Another used car is not unlikely to need repairs sooner or later.
Now that I notice that you have the petrol car, get a quote for the chain from the dealer and the independent, and then decide. It sould not cost you as much as the diesel car because you will not need to pay for the injection part.