Hi folks,
Just MHO...
Making a diversion landing in a smal airfield chanrging £10~20 is no big deal and it won't break anybody's savings. Except for the good-will factor that was mentioned before, I see no other reason why such an airfield should give people a discount since it's most likelly no more expensive that the original destination anyway.
But occasionally a large airport may be the best diversion option. Human nature being what it is I'd prefer not to worry about the impending 3 figure bill on top of everything else that has already gone wrong or that is likelly to go wrong in the near future.
On the basis of that I do think there is a very good reason why expensive airports should accept GA diversions at no cost.
Mak