Thanks for the feedback guys. I'm am beginning to think this has been caused by water wicking down into my boot via the sock.
I have some gaiters but have never used them; maybe now's the time! If the ground is wet I usually tuck my trouser bottoms into a long pair of woolly socks but I think the grass may have been a bit longer this time and has probably wicked down.
I have been using G-Max to proof the boots after they get wet by the way but I'm never clear on how to use the stuff because the symbols on the side of the bottle suggest that you apply and then wash off which seems like a waste of time to me (i.e. it doesn't have time to do anything before being washed away).