James,
I had a look at this link yesterday on pp, but didn't get a chance to reply.
If you go for those harnesses you will need to fit eye bolts to your car, which isn't easy if you want to retain the standard seat belts.
I would also recommend that you go for a good make, and don't scrimp on harnesses as they are there to save your life.
Lots of people (including myself as we were the first to fit them) have
THESE SCHROTH ONES which allow you to move forward when needed, but locks as soon as any G forces are detected.
These are fitted in the same way that Leighton explained, but you fit an extra box which triggers the G mechanism into the heated rear screen wire in the boot (from memory). Well worth the money.
If you want a fixed one then I would go for either
THIS SPARCO ONE or
THIS ONE
Just found
THESE SPARCO 3 POINTS on ebay