I think that you will find that you can get at the allen bolts that hold the latches to the frame (round the back) with a bit of care and that way you would be able to get them off the bike for better access.
all sorted, removing catches from frame worked a treat, had to break locating "safety tabs" but screwed back on with thread lock so they'll not be going anywhere on there own.
Think if washed and made sure removed all grit from arond sliding handle would not have had the problem in the first case.