Thought it must be a fuse, and sure enough F28 which supplies the indicators and rear wiper was blown. Fitted a new one and it melted instantly I turned the ignition on 
Disconnected the rear wiper and it was OK again - so there is a short on the wiper unit somewhere. Took it out yesterday, and then started studying the wiring diagram in the BOL. After about an hour of turning it various ways up and down, and I still can't figure it out - mind you as it suggests that there is a direct short to earth via the gearbox in certain positions, I'm not sure I believe it anyway 
So.... I presume that there is a fault in the gearbox causing a short as I've locked the motor before when it was seized up and that didn't melt the fuse, going to strip it down one evening this week and have a look, but I could do with a copy of the pukka wiring diagram, ahem, Jim if you are listening..... 
As it's out, might as well try and have a look to see if the relay needs repair as per the thread above, or fit a new one - if anyone's got one lying about they'd like to exchange for a few beer tokens
Citroen want £30 FFS!!The wiper arm is also broken - the plastic has split around the shaft, leaving the centre metal boss on the shaft (still got to work out how to pull that off!) - this is the later type which only takes the specific contoured Valeo blade - Citroenmad did you change yours the other way recently - do you want to part with the old wiper arm???? I'm pretty sure I could glue the arm back to the bush, but the Valeo blades are dear (well compared to the flat blades on the front anyway....!)
Of course, removing the wiper leaves a huge hole in the boot door, which will fill up with water. Attempt one with a roofing bolt, penny washer not quite big enough, and some gaffer tape was not successful, so I went back and rifled through the oddments box and turned up a set of toilet seat hinges - a nice big washer fixed on a pin, with a nice rubber washer to suit. Perfect
My Xantia is now OFFICIALLY a toilet!!
The wiring diagram is wrong - the centre wire going to the relay is common, and is either connected to power (to park it) or earth (presumably to give it equal potential across all parts to make sure it doesn't move on its own when not required). There is a copper disc on the back of the gear wheel that makes the connections, and power is transferred onto / off this via three wiper blades - and this is where the problem is - the end has broken off the main power feed thus putting it out of position and allowing both power and earth to be connected at the same time, hence blowing the fuse! I suspect this is also why it wasn't running smoothly when on self park, rather than switched on. Surprisingly I managed to tin the blade, so then cut a bit of copper, shaped it as a new blade and soldered it on. Putting the wheel in different positions, I can't get a short to earth. Result! Just got to put it back together (it has shipped rather a lot of water and is pretty manky inside again....) then refit it. Oh, and glue the wiper arm itself back together 
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