A scammer has been knocking on doors in Beddington and claiming he can fix roof leaks.
In one incident he went inside a house, went into the loft and came down with a piece of cloth he deliberately made wet and claimed it proved there was a leak.
He quoted £1,200 to fix the issue but the intended victim did not fall for the trick.
Officers say this is a common trick used by con artists and have heard tales of other similar incidents in the area recently.
Police advise people not to allow any unknown or unexpected callers into their homes and not to do business with anyone who cold-calls either on the doorstep or over the phone.
Sutton Council runs a safe trader scheme where local businesses are vetted by the trading standards team. Details can be found by calling 0208 770 5070 or visiting www.sutton.gov.uk/ssts.