Thanks for the replies so far. You've been kind of echoing my thinking really, i.e. that although it's some extra expense now, which is not ideal, if my water pump fails in 20K miles then I'll be kicking myself because I'll have a lot of labour cost to change it, whereas the labour cost to bung it in with the cambelt change will be negligible.
I had a look at the online service manual and couldn't see anything about this. All it shows is replacing the water pump as a unit, along with a gasket (I'm assuming a new pump comes with a new gasket ?). Couldn't see anything about o-rings or a by-pass pipe, where do these fit in ?