In case anyone else has the issue it turns out that changing repleater unit is not difficult. Getting the power adjust to work is more difficult. I think reassembly is not simply the reverse of dissambly and the mirror needs to be refitted and the gears aligned before the shell is clipped back on. Anyway, in the meantime it will now pass its MOT I assume and the mirror not power adjusting is on the driver side so it isn't a real problem.
The repeater unit was full of water because someone at some stage had clipped it and done just enough damage to let water in. Oh well.
