Alex,
I’ve not done Zopa, but have played around with Prosper, similar type of social lending site. Be very careful about reading the fine print. From the Zopa site, “In 2007, lenders have been getting their money lent out at an average rate of 7% pa, with rates in most C markets averaging above 10% pa (figures are after fee, before bad debt).”
Those figues are before bad debt! Depending on Zopa’s ability to make people repay back the loans, bad debt could really cut down on any rates you make. Prosper has been stating 9-13% returns, but many people have lost money on bad loans in the process.
Tuleja