Anne Rogers wrote:

So I'm completely stuck, I have no idea how to handle this. I don't feel
I have some kind of right to keep quiet until 12 weeks if it screws
other people around, but equally I want to keep living in a dream world
where when I'm dancing, I'm not pregnant!

I think you do have to tell him, and pronto. I think you
do owe him time to find another partner, if he wishes. Explain
that you believe you will be able to keep the commitments you've
made already, and will understand that he needs to take some
time to work on finding a new partner. Then ask him if there's
anything you can do to help with the transition. I would imagine
that there are decent odds you'll get to keep dancing at least
through the events you've planned, as it will probably take him
that long to find another partner.
While I'm sure it's probably not the news he wants to
hear, these things happen and it's not really anyone's business
to blow a gasket over it. If you strike a balance between
being regretful at the inconvenience to him and being surprised
but thrilled at the pregnancy, most polite people will at least
pretend to take it well ;-) And it probably won't hurt if you
throw in a comment or three about how he's such a good partner
that you're sure he'll find a partner eager to dance with him
in relatively short order.

Best wishes,
Ericka