It really depends on who you see in store. We have a discount code on the system to put them through for free, saves an argument for us.
If they dont put it through for free, call customer services and have a moan, 95% of the time they will credit you the money back onto your bill.
Awesome, thanks! I'll likely be popping into an 'east London' TMo store tomorrow, so if you happen to serve someone wanting a nano-SIM and being a business customer...