You're right, £3 would've been offensive IMO.
I remember giving £20 to the guys who fitted by double-glazing a few years back. I would have liked to have given more but was spending a fortune doing the place up.
Also bought lunch (not that sort – I was heading into town anyway) and gave a tip to the guy who did my cladding shortly after.
Am still waiting for the curry that a mate promised me when I laid a patio foundation for his outdoor office 2 years go. He reckons he's going to cough up the end of this month – nom noms.
EDIT:
do you have a few beers or couple of bottles of wine in lieu of cash?
great idea