Hello i am a solicitor with 20 years experience. I will try to help you with this.
The only sensible answer to this question is that there is no standard pro-forma contract that you could use. If you try to draft the contract yourself things could easily go wrong. You really need to see a solicitor in the normal way and get an agreement drawn up. This sounds like a potentially high value deal and it would be a mistake to try and save a few hundred pounds now that might end up costing you many thousands later. You can find a solicitor to you do this but follow this link law society find a solicitor. http://www.justanswer.co.uk/property-law/8sej6-hello-property-developer-process-helping.html?src=nql