One of Royal Mails many problems are the large quantity of fake first class large letter stamps available on ebay. Most of these stamps will get through the detection system they have. I cannot understand how they allow these fakes to be sold. Some even come in sheets of 66 stamps that Royal Mail do not even make. There is no security writing on the backing paper. I have looked at these fakes and often, there is a slight difference to the water mark on the Queens face. Some have a phosphor coating all over when it should be a central strip.
I use a lot of these stamps but now buy in a post office after losing some packages having bought what i thought were genuine stamps as advertised.