Good news, I checked the saw against a straight edge and well, it's out by between 0.05 and 0.10 mm (according to my cheap, not-necesarrily-accurate feeler gauge). That's not too much is it? It looked and felt like a lot though. I guess it was because I tested it in 2" thick pine. It's a dovetail saw after and will probably never see material thicker than 1". So I won't be too unhappy even if it never gets fixed. With a bit of wax it should work fine
One thing though, I was going to buy the new Veritas Tenon saw but this is making me think twice... Shame, it's a real bargain (Pax is more expensive over here).
Not too keen on antique saws (probably one of the only antique tool categories I don't like) as I've yet to see one that is completely straight.
-----------------------------------------------------
LV replied but couldn't give an answer as they couldn't quite understand the problem without a picture. Will send it tomorrow and see what they think.
It's really weird how no one else has this problem with their Veritas or fixed-backed saw in general, at least Google can't find anyone.