Gunnera Manicata Problem

I've got a three year old plant coming along nicely in boggy ground.But this year the older leaves have developed brown patches,gradually covering the whole leaf.I can't find any sign of pests,leaf miners etc.
Anyone got any ideas please?

 

The new leaves look healthy so it should recover.. but I have no idea what is causing this. I sometimes go out at night with a torch and check on a problematic plant just in case it is insect related.

 

Clutching at straws, but is hail stone damage possible or burn marks from hot sun through droplets of water???

As hans says though the rest looks very healthy so it should be fine