Hi Ian,
Hepco locks have a 2 or 3 digit code (stamped on the keys). I had a look at my panniers, but couldn't find the code anywhere on the lock. Motobins sell Hepco spares in the UK see:
http://www.motobins.co.uk/displayfinal.php?s=0&function=show&subs=186a&title=LID+%26+FRAME+LOCKS+FOR+PANNIERS+%26+TOP+BOX%27S+&model=&bike= and list a spare key and barrel, part no: 94568. This infers that there is some way of removing the barrel from the lock and it MAY have the lock number printed on it. It might be worth phoning Motobins to ask how to remove the barrel, if the number is stamped on the barrel, and whether they can supply keys by number. If not, it might be worth taking the barrel to a locksmith and seeing if they can cut keys for it.
If Motobins can supply keys, only buy 1 as you can get keys cut at 'normal' key cutting shops like Mr Minit or Timpsons, ask for a 'Silca LS13', this is the same as the Hepco key, except it's plain metal (no plastic cap) but will be a lot cheaper. While you're there, they can also cut spare ignition keys, ask for a 'Silca ZD 16RP'.
Good luck and let us know how you get on
Norman