Fraudsters posing as drainage experts from Dyno-Rod have been preying on elderly residents in Kingston.
Police are warning people to be on the lookout for three males claiming to be carrying out work at the back of residents' properties.
They then enter the property and distract the resident while one squirts water on the walls.
They then tell the occupier they have damp - and need to pay a deposit for a drying machine.
DS Vicky Washington from Kingston CID, said: "The victims now believing they have damp are conned into transferring money to a bogus Dyno Rod account for a deposit they need to purchase a drying machine.
"The amount can range anything between £1000 and £20,000.
A drying machine is then brought to the victims home.
"They then return on a weekly basis telling the victim the walls are still not dry and continue to charge for further use of the drying machine."
A Dyno-Rod spokesperson said its engineers wore a distinctive uniform and carried ID cards with a freephone number to call, but said they rarely called without an appintment.
The three suspects are white males in their thirties, and are believed to be driving a silver Mitsubishi Jeep.
Call Kingston police on 101 if you believe you or someone you know is a victim of fraud.
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