Mark,
Partnerships, like any other business arrangement, can fail, or work out great, BUT!!!!
If you are at all serious in doing this, go and see a solicitor, and get a partnership agreement written up to your exact requirements, this could potentially save you thousands of £'s and serious grief.
Do not copy any one else's agreements, it is simply not worth it, as a very quick example, you and your partner fall out, you could end up paying their Income Tax! This does happen.
A legally binding, written agreement is the only way to go, a simple agreement should only cost 400 600 pounds, it is the only way to go.
Regards,
Rob