Strangely, I found that insurance on cars over ten years old gets really expensive, to the point where the car is effectively uninsurable. I don't know why - but it was surprising, coming from a place where I insured a '95 Mazda for about $35 a month

Manufacturers stop keeping new parts in stock around that time, so spare mechanical parts and/or body panels begin to become scarce.

There's no point in paying for comprehensive insurance on a car that old. You only need liability.

The values I quoted are for third party insurance (akin to liability in the US)

I think its both. Insurers are by nature risk averse. I suspect, from your comment you also have no claims years? Still £900 is a lot.

Ah, managed to miss that bit - yes, I do. In that case I'm surprised your quote wasn't more!