Say house is £300,000; I could pay £150,000 deposit, and get mortgage for £50,000; friend might pay 10,000 deposit, and get mortgage for £90,000.
Hmm. When you say investment back, I assume that would involve deposit plus amount of mortgage paid off by each person over the years. Any profit when house is sold would involve sharing on the basis of what each person had invested (i.e. total deposit plus mortgage payments worked out as a percentage0). Does that make sense?