Insurers don't like insuring anything they deem to be bangers and scrap value, try a quote for something much newer with a tiny engine like a C1.
The difference in insurance costs might be worth changing the car for.
Also the newer/dearer the car, the cheaper the quote. I would imagine insurers think if you have a £400 banger you don't care if you crash it. Buy something 4 times that amount and you're going to be a bit more careful.