Asked an electronix wizard friend at the gliding club, he reckoned no problem so long as it's been unplugged for 30 minutes or so.
so, it's been unplugged for a couple of days and I tried to get the top cover off. Took out the four Torx screws (two each side) and the two small ones at the back, now it's free at the back but something non-obvious is holding it at the front.
Does anyone know what I need to do next to get the cover off? At the very least I need to remove whatever is rattling inside and use the other set of speaker outputs. So far as I can see looking through I should be able to send the broken bit to Arcam and get a replacement, and it looks to be attached to 'the works' by a very thick wire so there shouldn't be a problem replacing it.