If you ask maths teachers Fergus and Judith Wilson the answer to a simple equation they'll tell you off pat, writes Christopher Browne. But ask them how many properties they own and they may struggle to reply.

It's certainly more than 700 but just how many more depends on what they may have bought in the past week. Sometimes the former school-teachers buy 20 or more properties a month and in one year averaged almost one a day. At the last count they had 746 in their portfolio.

The Wilsons from Ashford, Kent, are two of the UK's most prolific investors. Their assets are worth more than £250million, their personal wealth is estimated at £190m and the couple have featured in the Sunday Times Rich List since 2003.

Their aim is to own at least 1,000 properties.

How they started