I would personally get a compression test done on the engine if no codes are showing. Those figures on mpg and power are not right with this engine. This engine can suffer lspi which could be the case for all symptoms reported or a head gasket possibly but not likely. I would personally pay for the compression test to be performed to rule out this first.Apologies, I have found the invoice for the work and it was camshaft sensors.
The codes I got were from my (cheap) OBDII reader, I guess the garage would have more detail with Vauxhall-specific diagnostics.