Thanks for the comments, gents. JT - it's not really a mystery how I make the "rust", just a mystery that it seems to come out well - most of the time
I bought some artists acrylics at a closing-down sale of a local art shop. I use the burnt umber with a touch of the cadmium orange - it's very bright, so you don't need much to produce a nice orangy-brown. I tend to keep the mix dark. Then I sprinkle in a little ground turmeric - the one I use I pinched from the kitchen, and is very finely ground. Mix well.
Finally, I dilute the resultant grunge with windscreen washer fluid. It's important not to put too much turmeric in, or the finish looks like the rust is the result of years at the bottom of the sea. Also, it's important not to over-dilute, so that the turmeric clumps. I use red oxide car paint as undercoat.
Once the lumpy paint is dry I give it a wash of very dilute Railmatch light rust acrylic, then with a less dilute mix touch where water would gather and newer rust occur. A finishing touch is to mix a little light rust with Railmatch oily steel to make a dark metallic rust which I dry-brush on edges to highlight them. I use a Derwent Inktense watercolour pencil in burnt earth to put in rust streaks - lick the end and you get a sort of thick paint - and a brown tongue
Other painting tips can be found here http://forum.gn15.info/viewtopic.php?t=3723
Now, I d like the team's comments on my latest attempt at ground cover. I want to grass the top of the slope, and I've bought some Heki grass mat. As ever, the light green I really wanted wasn't available, so I had to make do with mid-green. I've laid a strip down - not stuck as yet as I would like to know if it looks right. If I use it I'll make the edge a bit more ragged.
The alternative is to stick lots of individual clumps of Woodland Scenics grass and other vegetation over the hilltop in a random scatter. The bushes are Noch HO trees - a tip that has appeared on the Forum. The 'tree' poking over the fence has had the trunk extended, while the little 'bushes' are branches cut off another tree. I have a couple more, and might stick them in somewhere. Anyway, what do you all think?