Thanks for all your replies.
It's now two weeks since my original post and I'm feeling slightly more in control. I' still have problems wih interuptons, but have realised that a lot of them are related to tasks that I have previously delegated. I think I'm going to apply the 2 minutes rule more rigorously to the interuptions and if I can't deal with the query in 2 mintes then make an appointment to continue the conversation later. Perhaps I'll block out say an hour each afternoon to work my way round my collegues dealing with all those conversations in one go.
I think unstuffed has got it about right...
"So, my recommendations are:
1) Schedule interruptable time, leaving the rest sacrosanct.
2) Book a conference room for some intensive work times if possible.
3) Ensure your hard edges are really hard, and that you're not turning want-to's into have-to's.
4) Delegate as much as possible: you're supposed to be helping the juniors learn, after all. "
I'll be working my way through your suggestions!