Any electronics boffins?

[Andy Clap]

Cut it out - there may be some colour underneath.

Good idea mate, there was just enough left to identify it as a diode (though sadly not enough to rate it).
Guess it's to protect against using a PSU with the wrong polarity, in which case if I make sure I use the correct PSU I should be able to stick a bit of wire in place of the diode?!  I'll let you know if I get electrocuted or the house burns down!