One of the minor annoyances of the iPhone 5 is that is uses the nano SIM format, and replacement SIMs are in short supply if you're not buying on contract through a major telco. Amaysim has confirmed it won't have its own branded nano SIMs until November, but the MVNO is offering to hand-cut replacement nano SIMs for any existing customers who have purchased an iPhone 5 outright.
The offer is detailed on Amaysim's blog. It only applies to current customers, but there's no charge, and the SIM will be sent by Express Post. You can also try cutting your own SIM to fit, but I must admit, I'd rather the phone company took the risk!