pred said:-- each would get about £400 on top of their £1000 investment, or £4000 per £10,000, are my sums correct? is this better than banks or not?
Your sums are correct and a 40% return over five years is roughly equivalent to 7% APR. I think that's better than you can get from a bank at the moment.
The unknown in the equation is, as joe-90 keeps reminding us, the final value of the house. If you only get back what you paid for it, your profit is only 20%, which is 3.7% APR. Not so good.
The trick of course is to be careful where you buy, but you're the landlord so I guess you know that already. Eggs and grannies come to mind. Don't forget the small print. You'll be asking people to put their money into a scheme with no guaranteed return and maybe an uncertain maturity date too. They're going to expect some sort of a contract.