In reply to blackmountainbiker:
Are you sure the power line adaptors that the engineer plugged in are also for wi fi as some powerline adaptors are simply to run your broadband from one point to another via your ring main in the house.
the fact you can get wifi out to your caravan but not upstairs could be as a result of thick walls in your house? favourite killer of wi fi signal and therefore speed.
wi fi can travel further and clearer through a line that has less resistance like thick walls.
If the powerline adaptors are just for Ethernet then try and buy a separate wi fi extender and fit it at a convenient point in the house.
One way to be sure is that if your devices perform well next to the router but poorly in other rooms then its the wi fi that's the issue.