OCZ ZT 650w vs TX650M

Well I can find the TX650M for about £75. At a ~£6 price difference I would take the TX650M because I believe that the build quality is higher.
Corsair TX650M (£75 @ Dabs / LambdaTek / £77 @ Ebuyer / Amazon / £78 @ Scan)
http://skinflint.co.uk/682463

If you don't want the option to SLI then you don't need as much as 650W.

I'd suggest one, but most of the modular, lower wattage, higher quality units that I can find are not cheaper than £70. If you are interested anyway, say so and I will suggest some.

Yes please could you suggest some? I was thinking about going with the HX650 now as that has 7 years warranty.

The HX650 is probably a solid choice. One thing is that Corsair have recently released an updated version, which uses a newer design and achieves 80 Plus Gold efficiency. I think that buying an older model when a newer one has become available is a bad move, but the new one will probably be more expensive than the old one.

Another option:
Be Quiet Straight Power E9 CM 480W (£76 @ Ebuyer, Scan)
http://skinflint.co.uk/677396
Which is nice because it achieves 80 Plus Gold efficiency, and is very quiet (max fan speed is 920RPM according to BQ).

NZXT Hale90 550W (£78 @ Scan)
http://skinflint.co.uk/557337
Isn't modular but is a very nice unit. Probably one of the best below £100.

I noticed in one of your other threads that you said something about upgrading in the future. I would say that the power level of the GTX 670 is pretty high in this day and age for a single card and it is likely that power consumption for future generations will just go down. However even the Be Quiet unit I recommended would be able to support something even more power hungry, something of the level of a 7970 even. So you aren't really reducing your options for single cards.