How much to tip the milkman?

Our milkman has been a star throughout this period of bad weather.  He's never not delivered, even on days when we didn't make it to work, we got our milk.  I'd like to give him a Christmas tip, but money is really tight.  We're not spending more than £20 on anyone, even family.  So how much should I give the milkman?  We spend just over £12 on milk per month, so would £10 be appropriate?  More?  Less?  I don't want to insult him by leaving too little, but at the same time, I don't have much to spare.  Any advice? 

TIA!

i think a tenner is fine

I agree. We don't have a milkman, but I give a tenner (plus booze) to the postman and the bin men.

Our milkman has been the same and we found out he comes around about 3:30 when he delivers in the morning!  Even though we had a foot of snow he made it every time!  We left him £10.

I wish I had had some US expats as customers when I worked on a milk round, we never got tips from the natives! lol