Based off of the limited system info you posted im 99% sure that Liteon PS-6451-5 PSU is more than fine. I would venture to guess your entire system inst pulling over 300 watts. The 6870 pulls about 140 watts give or take a few watts.
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-...
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.
James is right - modern PCs use much less power than most people think. Read this recent article on power consumption in three modern PCs: http://www.silentpcreview.com/article1245-page1.html
The GPU mfrs spec their PSU requirements with some extra headroom because they know there are a lot of shoddy PSUs on the street.