Dear Mr Jones, 

Both my wife and I are very happy to give you positive feedback for your two visits. We both required a number of things to be set up on our respective computers and have been pleased with the work you underook. You were punctual ( a rarity), worked very efficiently until the computers were working well, and made sure we both understood how to undertake simple self-corrections when required. We thought your work was good value for money and we shall certainly call on you again when (and it will be when!), we need your help again. 

Yours sincerely 

Peter L.