If its a big project i would suggest getting a proper shareholders agreement drawn up. If you're looking at a real basic agreement these are normally between £1500/£2500. In the meantime you could get a Heads Of Terms agreement put together which basically outlines each parties duties etc while the shareholders agreement is being done, just a thought...
If you want some quotes from a few companies drop me an email and i'll pass you across.
Best Regards,
Barry