Bob emailed in asking for help identifying this mystery butterfly, which he can't find in his books. My best guess (from consulting ukbutterflies.co.uk's page on Whites) is that it's a Small White (male) that being the closest match I can see. It's sort of a cross between that and a Black-veined White. I'm a long way from sure however and I suspect there are knowledgeable people out there that can supply a swift and clear resolution. So please leave a comment here, tweet, or email if you know.
Update: Thanks to @BC_Suffolk and @SuffolkNature who both responded very speedily via Twitter to identify it as a Green-veined White. Apparently Suffolk folk are particularly hot on their butterflies! What threw me off the scent is that on that favourite reference page of mine the male is pictured with no spots and the female two spots, whereas there's very clearly one spot in the picture above. However plenty of other sources show the male with one spot, e.g. courtesy of Keele University Arboretum.