Suitably chastened by my experiment, I decided to revert to a more conventional approach ... the use of flock and PVA glue.
Firstly, I carefully applied PVA glue in and around the area covered by the marsh and sprinkled a layer of green flock (K&M Trees No.1 Grass Green) over it. I then let that thoroughly dry before using a desk vacuum cleaner to remove the excess flock.
I did this twice in order to get as thick and regular a coverage as was possible.
I then applied some more PVA glue to any gaps or areas where the initial flocking was rather thin and sprinkled some darker flock (K&M Trees No.7 Dark Green) over it. Once that was dry and the excess flock had been removed, I repeated this process, this time using a lighter flock, K&M Trees No.3 Light Olive flock.
By this time my marsh was beginning to look much more as I had wanted it to look, but the vegetation seemed to be a bit flat, and I decided to take a gamble and add some green clump foliage that was home-made from plastic foam.
This was made by soaking a couple of small pieces of plastic foam (mine were taken from the back of a pan cleaner) in a mixture of dark green and black acrylic craft paint and water in an old jam jar overnight. The excess fluid was drained off, and the foam was squeezed to remove any that remained.
Caution: This was extremely messy and was carried out over a bowl that could be washed out afterwards. Keep plenty of paper kitchen towel at hand to wipe off your hands as it is very easy to transfer the paint to everything around you!
The damp plastic foam was then placed on a small tray covered with a thick layer of kitchen paper and allowed to dry overnight. I carefully placed my tray near a radiator to help speed the process. Once the foam was dry, I tore it into smaller lumps which I then placed in a very cheap coffee grinder that I had bought for this purpose. Once it was turned on, the blades in the coffee grinder quickly chewed the plastic foam up into small pieces, which I tipped into a small plastic bag. I then very carefully cleaned the coffee grinder to remove any traces of the green foam!
The green plastic foam was glued in place using PVA glue. I used an old syringe to make sure that the PVA was placed very precisely and the small pieces of the foam were applied to the blobs of PVA. I then allowed several hours to pass to ensure that the PVA was properly dry before removing any excess pieces of foam.
To make sure that the foam remained in place, I dripped a mixture of 50% PVA and 50% water (with a tiny drop of washing up liquid added to it) on every clump of foam. I used my trusty syringe to do this. This had the effect of both reducing the chance that the foam would 'shed' during use and hardening the foam.
I’m now satisfied that the marsh is now finished, and I can move on to the next stage of building my layout.