Thanks for the kind comments guys.
Getting there. Its great to be able to share the begining, middle and hopefully end of the build, it feels like we're all involved and on the journey with you. 
I found Richards threads very useful. I hope that others might find mine useful.

Also the wagons arrived. They look great! Thanks again!
Like the progress so far, looking forward to seeing how this develops.
+1
Richard is the man who brought it to my attention. I've ended up with two recipes for the gloop, one of model bases and one for terrain boards.
I use 1 part PVA, 1 part paint and 1 part sand. For models I use a variety of grain sizes, up to some larger pieces that are maybe 5-6mm.
For the board I don't use such a wide variety of large grains. I did in the past and I have found that individual terrain pieces don't sit on it well. I'm currently experimenting with Kiln dried sand from B&Q (I think I'd used sharp sand previously). We'll see how it goes!
In the image below you can see the results of the filler and PVA glue. It's hopefully help to strengthen the boards and prevent it from damage. The PVA will help to make sure that the boards don't absorb as much paint, and I should now be able to spray paint them.

As you can see the gloop goes on quite thickly. All six boards have been covered now. I've deliberately left the depression untouched- this means that I can finally decided whether to turn it into a marsh or a small body of water.

I won't be able to work on them over the weekend. I'm in two minds about the next step- I'm not sure if I should give them a spray undercoat to 'bind' everything together and add extra protection, or just start painting.
Kind Regards
Andy