Thank you both for that - I had a feeling most roses were grafted, I guess I'm thinking of the odd species rose as well.
I've just been out to have a quick snout before it gets too dark, and the new shoots appear to have five leaflets (I'm assuming that the stock would be something like sweet briar, which has seven?), so I shall cross my fingers and hope. Mind you, if it does cark it, we're practically on first name terms with the staff at our local nursery anyway, so I'm sure they can find me one...
If it turns out to be the rootstock I'll shove it in the hedge where a bit of extra prickle will not go amiss!