Thanks everyone. I was thinking 6-points but the Schroth manual implies that this would be dangerous with S-type seats:
'Never use the SCHROTH Racing Harness unless the racing seat is designed with anti-submarining holes.
Never run the anti-submarining straps over the front edge of a factory seat down to the floor. Such routing does not provide the desired anti-submarining effect, and in fact encourages submarining, which can cause severe injury and death.'
The Schroth website also has some interesting info, although it has started to persuade me that I need one of their 4-point harneses with 2" lap belts and Anti Sub Marining Technology.
available here
These just happen to be twice as much as the Titon 6-points I was looking at originally! I guess seat belts are not something to skimp on but I still don't want to get sucked in by the marketing BS.