I've found them extremely reasonable. In the summer we dropped in on the way back from further south because the forecast for clouds over the uk were too low to risk.
Once we landed, a pilot had just crossed from blighty and gave a good report of cloud base, we re-checked the TAF/METAR's and then decided to go back immediately.
The guy on the desk had overheard the conversation and refused to take our landing fee! Now thats what I call reasonable (since we had done a planned divert not an en-route divert).
I think if you ask politely you may get a better explanation.