Sounds like a great idea to have a themed hackday.
Really active Health community with
http://www.meetup.com/Health-2-0-Manchester/ Recently they've been running Service Design workshops that would be a really good precursor to any future hackday.
I'm sure Vim (spaceport) and the Service Design team could be persuaded to get involved.
Perhaps the MSP Greenheys space that they run the workshops in could be utilised. Idalia Dawidowska, the organiser is really approachable.
I was talking to a local gov't backed organisation last week who are looking to finance 5k health projects, through a scheme they're trying to finalise. So perhaps that could be incorporated into a hackday i.e. continuation of the projects, rather than all the 'hacks' dieing off.
Madlab has just been refurbished so that could be a good small venue if you think more 50-100 is likely.
NHS Choices in Leeds are often at NWRUG so perhaps they'd have motivation to get involved also.
hth and let me know if I can add anything or intro anyone.
Cheers,
Ian.