Thank you for the advice all. Upon reflection I definitely that that I'm going to reinforce the bottom of the board. A wee bit of extra protection. I may also reinforce the sides as well.
I was down in Glasgow for a wedding so I never got the chance to work on the board until today. Managed to finish sanding the river and path. I think the river is deep enough, although I do think I've went a wee bit too deep with the path. I suppose that I can fix it with a bit of filler.
After working on the larger hillock, I decided that the board was still a little flat so I added a couple of extra wee bluffs. I've glued them down. From the pictures you can see that they require a wee bit of polyfiller and some aquarium gravel as scree.
I am hoping that the small depression that the ruined cottage is sitting in will disappear once I've used Captain Blood's patented Gloop.
Cheers
Andy