Thanks all for your kind remarks.
The hill was made from offcuts of the pink foam I used for the benchwork. I used 4 layers of 1.5" in fairly random shapes. The bottom layer was cut to fit the space and then each succesive layer was offset about (very much about!) 1.5" in from the previous one. That made for an 80' hill in N scale.
I used blobs of hot glue to stick the layers together but next time I will use something else (Elmers?) that I can coat the whole layer with - the seams had a tendancy to open up in the next stage. If you look carefully at the picture, you can see some of them.
Next I blended the layers with a surform held at about 45 degrees and just hacked at it. In the couple of places I couldn't quite get into with the surform, I used the little sanding drum on a Dremel. This got me to the shape very quickly but left a rather torn up surface. As Slow said, hills don't have to be too smooth but this was REALLY rough so I finished it off with a quick run over with hand-held 100 grit sandpaper. I may have overdone this but I was going for an old, rounded sort of a hill. None of this was very exacting work and the final shape (except the footprint) was as much of a surprise to me as it was to anybody else

It took me about 2 hours to this point. I gave it a quick coat of some brown spray paint that I happened to have lying around and had dinner while it dried.
Earlier in the day, I had picked up some of the Woodland Scenics learning packs and I got to try a couple of them on this project. First I covered the whole hill in the basic mixed green fine turf. This pack (the landscape pack) also includes a brown undercoat which I used but this was probably redundant with my existing brown paint. I would skip one or the other next time. The Woodland Scenics undercoat was rather thin and didn't cover the pink foam very well so I would probably stick with my spray paint. It would probably be petter on plaster. I threw on a few highlights of the different colored turfs and gave the whole thing a coat of the glue.
While it was drying, I made up all the deciduous trees in the tree scenery pack. These are RIDICULOUSLY easy. After a little measuring and thinking, I discarded the two biggest trees as they were about 80' tall in scale which is not impossible but they would make a lot more sense in HO. I may yet put them on the back of the hill and let them just peek over the top.
I punched some holes with a small screwdriver, put a dab of hot glue on the base of each tree and planted them where they looked right. I planted them in descending size order, filling in between the big ones with smaller ones which seemed to work out for me.
I have two more of the WS packs - roads and rocks - so I'll probably have a play with those tonight.
Up until now, my main artistic talent was being able to pick up a paint roller by the right end two times out of three

so I am very pleased with the results so far. I can see where it could have been better but I think I made a very believable hill in about 4 hours. My wife says that, had she not seen the intermediate stages, she wouldn't have believed I had made it.