Thanks Altoid,
This is the actual Message that a customer receives, not very helpful.
This invoice has alreaddy been paid. For more information, please contact the merchant.
Your purchase couldn't be completed
There's a problem with the merchant's PayPal Account. Please try again later.
I have also used my wife's paypal account to purchase from my site with the same result. As far as I know I have made no modifycations to the site, it just seems to have fallen over.
I still wonder if the date has any significants, with updates coming out the day after the last successful purchase. I have not updated as my site has been modifyed using Gary's book 'Designing osCommerce 2.3x with 960 gs' and did not know if that would cause problems.
I have re-installed the PayPal Standard module, but that has had no effect on the problem, it is dated 2008, I cannot find anything newer.
As I said I can manually send an invoice and receive payment, so my PayPal account seems to be working. I don't know what to do.