Just an update -
Last weekend (as mentioned in some other threads) I managed to get the bike through some pretty serious flooding, on the same ride the roads were wet/muddy so the bike did get a bit of a covering - and I had the top box on!
Yesterday, I tried to get the sodding thing open, and it was as sitff as hell, so I had a run over to the dealer (J&L Motorcycles in Carlisle) and asked them to take a look. I must confess that every problem I've been to see them about (I've three Yamaha's, TTR, Fizer and the Ten) they have resolved on the first visit, in my opinion they really are top notch. Fountain of knowledge the lot of them.
Anyway, they squirted some, what they described as 'Lock Oil' into the barrel and working of the lock - got it home, so it had had a chance to work it's way in - and the lock is better than new, smooth, and it doesn't feel like it's going to snap.
Maybe part of the problem here is that the locks just don't have enough lubrication ? I know when I stripped down the lock on the panniers and lubricated them they were 100% better, a bit rough now, but they have been half way around the world.....