Forge World is experiencing problems with online payment processing.
From their website:
We are currently experiencing a technical issue with our secure payment provider which is hindering successful completion of credit and debit card payments via the Forge World online store. We are working to resolve this as quickly as possible, and will update you once our system is up and running again. If you experience any issues while ordering through the Forge World website please call our Customer Service team on 0115 916 8177 if you live within the UK; 00 44 115 916 8177 from much of Europe or 011 44 916 8177 from the US and Canada.