Thanks again, Dominic. I already had Extended Terrain ticked but this didn't help a lot for some reason. However, the adjustments suggested in the link you supplied have made a big difference. In fact I think that restricting the distance of visibility right up to cruising height actually makes the ground look more realistic. I can now see that it is unrealistically sharp from high altitudes at the normal setting. The downside of restricting visibility distance is that you can't use real weather but I am thinking of investing in the complete version of FSUIPC which, as far as I understand from the link, allows you to make the adjustments and still use real weather.
Also, I meant to email and say, Ground Environment is much better than default. Some areas around add-on airports look no differentt but once you get out from there, the differnece shows more clearly.
Ian