Hi Joe,
Thanks for the reply. The fun that I had was that I couldn't login, so I was having difficulty changing the skin! I've done some experimenting & have found something rather strange...
I copied a brand new v2.4.0.8 installation onto a different domain, but used the same machinekey & connected to the original database. Everything worked fine! That made me think that your comment about skins might be where the problem was. Infact, my web hoster suggested it might be a similar problem & directed me to this blog: http://blogs.msdn.com/b/webdev/archive/2014/09/09/farewell-enableviewstatemac.aspx.
However, after backing up the old 'data' directory, I deleted it & copied a fresh data directory in it's place. Essentially both instances on both domains were then identical & are using the same database. The new domain works fine, but the old original one still has the same issues! Both are running in the same isolated application pool, ASPNET 4.5, IIS 7.5. I'll get back to my domain host & see if they can shed any light why this might be occurring and will let you know if I find a solution...