CT1651 is right in that the PSU is working harder to supply the power your system is demanding with the new GPU. On the other hand, your system is running. Are you experiencing any system stability issues? Locking up, reboots, shutdowns, etc? If you're not having any stability issues and the heat/noise from the PSU is acceptable to you, then you don't need to get a new PSU.
A less expensive option than the M12II in the UK is this XFX Pro 550W: http://www.scan.co.uk/products/550w-psu-xfx-pro-series-core-edition-p1-550s-ukb9-85-eff-80-plus-bronze-sli-crossfire-eps-12v-fan-at
It's actually made by Seasonic and based on the M12II PSU. I'm not saying you absolutely need a new PSU, but if you decide to go down that road, then this a good option for you.