I know exactly where you are coming from Si, I often do lots of work on "mates" cars. 95% of the time it is appreciated and I don't mind doing it as it is nice to help a mate out and save them a few hundred quid, but sometimes as you say, it backfires and people expect you to be availiable all the time 24/7, and don't appreciate that having worked hard all week, the last thing you want to do at the weekend is work on a poxy cossie for 6 hours in the freezing cold, pissing rain .

The worst experience of this is when I stripped my mates smashed cossie, ( in december in the snow outside) and the cheeky fooker complained that I had not done enough, even though he had been at work all day leaving me to rope my brother in as a second pair of hands
Then I lent him £100 to buy a 4 spoke Rs steering wheel, which wasn't a prob, the wheel was a bit worn but ok, and I said give me the cash when u get the chance- well, matey finds a MINT one on ebay, and asks me if I would want it instead of the £100 back, so I said ok, as long as it is 100% as I wasn't really after one. So a week later, I get presented with the FIRST wheel he got, after he recived the mint one and promptly bolted it onto his own car . I let it run cos i was too nice , but there you go.
Then, same mate blows up his cossie ( after being repeatedly told NOT to thrash it as it was running lean ) after I had spent 3 hours on the phone trying to get a RR session sorted, as it was the start of race season and everywhere was chokka, so I strip out the engine, take a week off work to build the new engine, get it all built bar a few parts we couldn't get untill saturday, so by sunday night it was all good to go, as it was bank holiday monday I planned he could help me put the engine back into the car, but he decides its more important he goes to alton fooking towers instead so I spend all of bank holiday monday trying to put the engine back in the car ( it was a 'mare, nothing would line up or go on ) - Not so much as a thankyou needless to say I don't work on his cars anymore
but its not all bad, Me and my mate Kursedman built his cossie engine, and when it made 370BHP on greens @Engine advantages it all seemed worth it