Thanks for the responses. Thinking about it, I'm probably being optimistic in saying it's only 30 years - I'd say it's more like 50+, from the hideous bright pink bath & sink.

I've measured the pipe, it's roughly 15mm externally, which doesn't tell me whether it's imperial or metric, but does tell me it's not 3/4".

Based on the responses so far:
1. It's probably not safe to assume that 15mm & 1/2" are interchangeable.
2. Given the small difference in the size though, a new 15mm tap will fit in the hole in the sink for the existing tap, even if it is 1/2".
3. Given the age, my best bet for the flexible hose is probably 1/2" pipe to 15mm tap. If I'm wrong on that, it's not a large loss (about £3 from a quick look online).

Pls shout if this doesn't make sense.