I use Darren Barrels (aka Darren Drums) that I line with thin neoprene. These are very water and mud resistant, but not suitable for underwater use. These are durable and relatively inexpensive. I believe they are made on your side of the pond.http://www.solentplastics.co.uk/marine- ... egs-drums/http://www.hitchnhike.co.uk/acatalog/ex ... rrels.html
Depending on how wet or muddy the cave is, you will need another barrel for towels where you first dry your hands and strobe barrel, before you open the strobe storage barrel. If these barrels are too flimsy, then look at Pelican or Otter Box alternatives ($$). A further alternative is building your own dry box or tube with Schedule 80 PVC tubing. Using an o-ring seal for the "lid", these are quite water and pressure proof. They are also a bit heavy.