Welcome, glad to see someone else from the UK
Another option is to film your trips - using a cheap camera from Ebay (£15), or your phone in a windscreen mount - and a gauge of some kind. That way you can get a record of your average and instant readings on any journey and compare them. As EM says any gauge, including the built in ones, can be out as they all "guess" what is happening based on signals from the ECU.
There are some videos
here.