i dont see £20 every few weeks as a great problem tbh.
you cant really complain about that amount of cash when he spends so little on other things like alcohol.
surely you buy magazine and more at hairdressers/salon, than his habit does.
there are also many many worse habits.
forgive me if i'm wrong, may have read this wrong.... but does he only disappear for 20 mins in a night?? i dont understand how going outside for 20 mins after a hard days work(where you say he is doing well and pulling his weight) is going to constitute him missing out on family life?
surely there's a fair few hrs in the evening between returning home from work and going to bed.
is he not entitled to just 20 mins peace without being glued to your side?
if the problem is that he does this as soon as he comes in, then just slobbing out and not helping with the family stuff.....then why dont you agree with him that he has to wait until after dinner, bath and bed routine, and house all cleared up... then he can have his 20 mins spliff, and you can have 20 mins to yourself also.
the truth is you say you clam up when it comes to giving him an ultimatum.... i think deep down you are aware there are much worse problems and you are being a little controlling.
he has always done this, and you have always known he does this. how is it fair of you to decide its not ok for him to do it, its his body and his choice. you made the choice to give up this habit. not him... and until he is ready he will not stop.
try and come to a compromise perhaps in time he will give it up altogether. in the meantime, getting him to pitch in and resolve the issue around chores may help the relationship.