Expect to pay £60 - £80 or more for a good 700W PSU.
Many people like Corsair because either they are good PSU's, or they think that Seasonic are the OEM.
However, you may be surprised to learn that the only Corsair PSU's that currently use Seasonic as the OEM are the AX650, AX750, and AX850. The AX1200 and the new AX1200i both use Flextronics as the OEM. All other current current Corsair models use CWT as the OEM.
All XFX models use Seasonic as the OEM.
OCZ use a variety of OEMs including Sirtec, Great Wall, CWT, etc, but not Seasonic.
Be Quiet use Seasonic for their Dark Power Pro P10 models for 850W 1000W and 1200W. Most of Be Quiet's other models use FSP.
You can find the OEM for the model(s) you are interested in by using the PSU database link in my signature.