Thanks for your reply @Lennym1984
My car is a standard 987.2 S, so nothing particularly exotic or unusual. But it's in great condition, with very low mileage, plenty of options, and a full OPC service history.
Over the past few months I've seen lots of higher mileage, less well-specced cars on AT at considerably higher prices than the market value that flashed up when I was getting a quote through a comparison site.
I think you're right about the PCGB valuation, it probably is a little optimistic, and of course the cars on AT don't sell for their full asking price.
So you're probably right, the +25% vs market value will probably be plenty. £49 seems like a pretty good deal for the peace of mind. Even added to the main policy I just bought, it's a lot lower than what I paid last year and the renewal quote I got.