"a couple of months ago there was a lot of chat on the Computer bit of Prune about this - essentially the geeks reckon that as XP is unsupported you are wide open to cyber attacks and malware. "
I've been out of the MoD for some years now, but even then, with the end of Windows XP support imminent, the MoD had contracted with M$oft and was paying an arm and a leg for continued support for XP for a 2 year period after the official end of support to cover the migration to a new OS, certainly for the corporate intranet, not sure about operational systems.
That facility for continued support was also available to other large corporate users which is probably why patches were available so quickly for general users in the latest incident.