The best thing to do is fork out the price of some ladders and be available for your frail old parents, they would then not get ripped off.
If they don't want high ladders at their home ruining the look of their garden you could buy a large van, tax and insure it and tie the ladders to the roof and nip round when needed. It's a real pain in the arse taking them on and off but if you enjoy it you could go round all day getting £5.00 jobs changing lights, clearing gutters, fixing a moved slate (even though it's illegal and you really need scaffolding).
Many other trades have a fixed call out/appointment fees, many believe builders should not, that they can get free quotes that take up hours and emergency call outs with just a thank god you came.
Tradesmen arn't there to service the elderly community who have children that don't pull their weight.
I've known lots of tradesmen who are closer to elderly people than their own children.
Don't call anyone out unless you're prepared to pay, people mentioning £10.00 are mad. It would have been rude to give him less than £20.00.
£30.00 is probably worth taking the ladders off, £40.00 would have been respectful and anything above would be a sign you like the tradesman, are a good and prompt payer, want to tip him, and would like his services in the future.
Give him £10.00 and you probably won't see him/her again, even for a bigger job, they've marked you down as tight or someone who sees their job as unimportant.
A drink is just terminology, not to be taken literally, if it was a longstanding customer, that drink could be paid and added onto a later job but for a new customer it means I'm leaving that to your discretion, your parents could have ripped him off, this was a sizing up exersise for the future between both parties.
I think on this particular occasion he should have handed 10 or 20 back.