When it gets to situations like this I think “what can I buy for £700” and it’s usually not very much. Better the devil you know, as suggested above.
The only time I get rid of a car is that it’s gonna cost serious money to fix, AND I know there’s a list of other problems that will need fixing very soon and the car is really ready for scrapping.
However, if you want to change your car anyway, and feel yours is pretty much ready for scrapping, then it’s probably the right time to do it.
Autotrader has a really good search function I find, gives me an idea of what is available, locally, within my specs. A good starting point (even if most of the listed vehicles are already sold!).