There are lots of different computer models that produce forecast data. Each of these are run with different starting points (peturbations) and also at different grid sizes (coarse grid =quick to run but comparatively rough and ready to give an idea, fine grid=potentially much more accurate but takes a lot more time/computing power)
Metcheck uses just the gfs model with no human input - its just raw output made pretty. in fact several of the websites use gfs as it is freely available. GFS has been pretty accurate recently but it does almost always seem to overdo rainfall totals.
The met office have their own model suite, some of which isnt commercially available. They also have highly trained forecasters - I'd say if in doubt look to them!
Sorry to ask a silly question but why are forecasts on different websites so different? The Metcheck is always doom and gloom compared to the Met Office ones for our area. Where do they get their information? Why is it different? I'm starting to get quite interested in the weather but having no background on it I don;t know where to start. Are weather stations that you can buy any good? Thanks