It looks like the new HDD for the server is failing (a 300GB one!). Occasionally the IDE controller will lock up during initialisation - unplugging the HDD fixes this... randomly power cycling with the HDD connected will get it to boot 1 out of 10 attempts. I've spent the whole day backing up the data so when I get a replacement I wont have to waste time gathering data. Consider all events that occur on the server from now until the replacment (hopefully by the weekend) as non-permanent (i.e. forum posts may be lost etc...)

Look how I spent my day...