Okay folks, I'm going to contact the hostel people today. The dates I'm going to enquire about are:

The weekend Friday 28 Feb - Monday 3rd March

and

The weekend Friday 7 March - Monday 10th March

I think they can sleep up to 50 there (or maybe it was 30 - can't remember!).
Anyway, I'll get the details and report back. I'm not sure how we'll go about booking. It's possible I could maybe ask them to hold a block of bookings provisionally for us, and then people could phone up individually and make arrangements directly with the hostel.

So - last call - these dates okay...?