Not a bad idea Russ, have you tried it? how did it go down?
I would think £20 would be better taken on card then deducted from their bill.
OP Yes I thnk you can sense the lack of comitment in their voice tonaility. Very annoying, I could write at length on this and simular annoying things about how we can be treated by the great Brittish public.
One eg. was a landlord got me out to do a written quote, only to show it to her tennet to deduct it from her deposit but never had the work done, this women was quite happy to waste my time, not to mention what shes did to her tennet.