My take on this is to keep the tea and biscuits coming, in the morning a bacon roll or two each and at lunch maybe Greggs sausage rolls, a sad fact is if you look after them they're more likely to do a better job and will be more receptive to any questions you might have during installation, a tip at the end they won't know they're getting is also good but maybe too late. Maybe a crate of beer each, try and get a handle on what they drink.
Just as a service engineer I've had bottles of wine, trays of lager, gift cards, yes money too but for me the best was the food, Polish people set out a banquet for you at lunch, Chinese and Asians also used to do the same even though I was only toe and heeling a door or replacing a unit
My tip is look after them, don't complain about the dust, it's not theirs - they didn't bring it with them, it's your dust.
