Thank you! Yes, it was the global textures which I had on a medium setting. I have now adjusted to high and things are a lot clearer.
Although I have recently upgraded my graphics card I have an Asus motherboard and Intel processor which are now 4 years old (a very long time in PC terms) and I am planning to upgrade them and use Windows 7 in place of Vista. Hopefully this will improve things dramatically. I currently have 3mb ram but I see that modern motherboards and Windows 7 allow you to use a lot more ram so again I will experience better performance.
Regarding this forum, I can't find a search facility- if there is one this might save some posts as I am sure I will have a lot more questions- this is after all a complicated sim!