Thanks for all the tips everyone - thought I'd stick a photo of my first attempts here:
Made with a variety of tufts and aquarium plant cutoffs, much of which was embedded into polyfiller. I didn't stop to think at the time how difficult it would be to paint the base and filler with plastic shrubs etc sticking out of it, so they look a bit of a mess, but on the other hand jungle (or in this case 'generic dense vegetation') isn't neat and tidy!
Quite pleased with the first effort - the next phase will be to make up some bigger bases that the round ones can just drop into or not, pretty much stealing the idea from Ockman's blog