Having asked this question and received a very satisfactory answer, I now wish in hindisght that I had made a different choice for the layout of my subversion repository.
My repo layout is currently:
/trunk /Project1 /Project2 /branches /Project1 /Branch1 /Branch2 /Project2 /tags /Project1 /Tag1 /Tag2 /Project2
Of course now I wish I'd made the other decision:
/Project1 /trunk /branches /tags /Project2 /trunk /branches /tags
So the question now becomes: how can I make this transition, while keeping my version history intact? Is there a way of doing that?