golf gti 1800cc turbo?

just purchased a 53 plate 1.8 turbo 85,000 miles. on test i thought the car was suffering from turbo lag in 4th. purchased the car did 80 miles no problems then experienced slow running problems diagnosed by a mobile mechanic for £65 as a temperature sensor , went well, runs well still, light came on again after a week , cancelled by disconecting the battery. any advice dont want to pay another £65.