Terry, according to your Aercus WS 3083 manual, make sure you have selected Relative Pressure before your enter the pressure that matches the local benchmark. If you are sure you are doing that correctly, then your easiest option to calibrate it is to repeat the process but this time add 1.8 hpa (or whatever number you want to use to try to make up the difference) to the local benchmark. You will need to play with it for a few days to see how closely it will track the MetOffice benchmark. It is normal to have some variance so don't worry about it once you get close. Unless your altitude is exactly the same as the benchmark, your pressure should be different anyway. Generally speaking, the higher your altitude is, the lower the pressure will be from the benchmark. Obviously if your altitude is lower than the benchmark, your relative pressure should be higher than the benchmark.
There is one other option, that I personally don't recommend; it works, but it's not as clean as what I just described. In WeatherCat preferences | Misc2 there is an option to Set Calibration Data. You can go there and enter an offset for Pressure. The offset can be a positive or negative number. What this does is add the offset to whatever reading your weather station hardware is producing. I don't like this method because it requires a calculation to change the reading on your weather station. I think it's simpler to just change the reading on the station to approximate the local benchmark.
I suppose this actually means that I am suggesting you do your calibration on your weather station itself, but go check the preferences setting just to make sure you haven't accidentally entered an offset there that is modifying the station output. Your offset should be 0.000 in the settings.
One final note, do not keep the Set Calibration Data window open longer than you have to. Whilst it is open, weather sampling and weather related processes are halted until the window is closed.