Baz, sorry mate - that was just a photo I grabbed from a discussion on the net about the exact tyre sizes that are being discussed here. Can't help you with real world measurements. I'd love that car though and I like the colour - there are too many red cars in the family at the moment!
The only option I can think of in a taller 17" for you is the M&H Racemaster, which is offered in a 325/45/17 size - the centre pair of tyres. Never mind the outside pair, they're an inch shorter than your current front tyres...
This is about a 28" tall tyre, so it may be too tall. It certainly wouldn't be too wide. M&H can't really grasp the concept of accuracy or consistency in the widths. Here's a comparison of the M&H 325/45/17 against a 295/50/15 tyre. Definitely taller (roughly 28" to 26"), but narrower!
They're a cheaper tyre than the Avon, but there would be no doubt the Avon would be a much more accomplished tyre.