The problem didn't exist in XP, and I got rid of Vista so long ago I can't remember if it was present then :-) I have been through all the relevant power settings and set the various lid options to 'do nothing' but this did not have any affect.
I have just read a long thread over at http://social.msdn.microsoft.com/For...a-60c9bcb81f57
which relates peoples experience of this issue with Vista. Within the thread is a temporary solution of disabling the 'ACPI Lid' system device within Device Manager. I've just gone ahead and done this and since Windows now knows nothing about a laptop lid the screen stays on even when closed.
Unfortunately this doesn't really solve the issue, just works around it so i'll keep investigating. If anyone else has any thoughts/experience it would be good to hear them.