Paypal have been tweaking their security requirements, and as a result, it stopped working in our online shop, even though all the listed security requirements were met!
After pulling out numerous hairs, and getting absolutely NO help from either Paypal or the hosting company, I decided to move the site to a new host, and in the process move to a much newer and faster server.
As I suspected, this resolved the security issues, and after a bit of tweaking, the shop is now running much faster too. I don't think I've even seen an online shop load this quick!
As usual when you make a change, there is always the possibility that something isn't working properly, so if anyone notices anything out of order or unusual, PLEASE let me know, so I can get some sleep at night again.
Thanks guys!