As it says on the tin…

Got a 7spd cassette (Shimano) on my retro road bike and it needs replacing. Was thinking whether I'd take the opportunity to upgrade to 8spd while I'm at it (well aware that I'd need new shifters). So would I need to replace the freehub body too to accomodate the 8spd cassette?

No.

8/9sp is interchangeable, but not 8 on a 7.

7sp cassettes are ridiculously cheap mind…. 🙂

http://sheldonbrown.com/k7.html

I just this minute found the Sheldon Brown link. Probably looks like I'm gonna stick with a retro 14speed machine – although I think that implies I have to stick with down-tube shifters as all the STI ones seem to be 8+ speed…

How old is the hub?

For the last decade or more (since 1998 or so) almost all hubs have been wide enough to cope with 8-speed. A 7-spd cassette fits with the aid of a simple spacer.

Hub is probably about a 1990 vintage, a Shimano 1055 from the age before STI shifters were available!

Was 1055 not 8s IIRC?

1990 though, second year of hyperglide, 7s most likely.

You can get away with 8 speed STIs on a 7 speed set up.

And 8 speed Campag Ergos shift 7 speed, same spacing.

Change the freehub body for an 8spd one? Used to be able to do this with older hubs. Its only a couple of mm wider at most.

As said above, you could change the freehub body and alter the hub spacers/dishing to compensate, I did quite a few many years back when some dick working on our stand at the bike show sold a load of 7sp wheels as 8sp.