I seem to have found a solution to my calendar diary requirement.
I've been looking for a simpler wiki (than TWiki) for personal use and after a fair amont of hunting around and wasting time trying things out I decided on MoinMoin. It's widely used (and I almost tried it myself once before) and it supports reStructured text as one of its markup languages so fulfilled quite a few of my wiki 'wants'.
It turns out that MoinMoin has a ready made 'out of the box' macro that generates a calendar with clickable days that take you to the page for the day which (as with any wiki) you can create if it doesn't exist already. That's a large chunk of what I want, if I can link some sort of appointments utlity to the pages I create then so much the better but that will be a bonus.