Hi Sha,
Thank you for taking the time to help me out, I really appreciate it.
I have just spent the last few hours scratching my head and googling as I had already tried your solution but it didn't work for me. I did think that I must have made some sort of typo and so I copied and pasted from your code which works fine in your examples but again, it doesn't work on my site.
I think that I have found the reason and also discovered that I may have opened a bigger can of worms than I thought. I maybe should have said in my original post that this site was a Joomla 1.5 site but I didn't think that it would make any difference as the .htaccess file is outside of the joomla installation.
Apparantly Joomla overwrites any HTTP Status errors and so any .htaccess rewrites for files not found won't be carried out.
I think that my best option is to try to find out if it is possible to disable the Joomla HTTP Status error handling and to use your solution but unfortunately it's going to have to wait until tomorrow as I am already in the bad books for working late again.
If I find a solution I will post it here.
My issue is probably still a long way off being resolved but you answered my question perfectly and I am very grateful for your help.