If you want to tip the assistant because you feel his hard work deserves that little extra on top of his earnings and you want to show your appreciation for his efforts, then go for it. I think it's a nice thing to do, especially if he is young and he will no doubt find this act of appreciation very encouraging. If, on the other hand, you are doing it because you feel sorry for him etc., then that's probably not the right reason, although I can understand why you would want to. As for amounts, �20 sounds a reasonable sum (it's a couple of rounds down the pub), unless it was a particularly long and arduous project and you might feel a bit more is appropriate.