Thanks for your replies all.
FYI, it's a good, rigid nearly black sink made of some sort of resin. £200 worth, so it ought to be good! Anyway, it's rigid enough to survive on a bead of silicone if need be, so any clamps that may miss aren't the end of the world.
I did use good blades. B&Q stock thier own 'P' blades and another brand, much more expensive. £6 and some change for 3 standard looking wood blades of about 75mm size. I marked the underside because the surface took the pencil better and a standard blade would then cut 'downwards' from the point of view of the finished worktop. I then drilled my start hole, and gently allowed the blade to cut it's way along; I didn't push it. Problem was after about 2 feet it slowed down and the smell of burning wood appeared. I did NOT have the rocking action of the blade selected. (Only because I forgot about it). With big (deep) cuts like this is it a help or hinderance? I suppose I should experiment.
Anyway, Weather permitting I'll redo this bit of worktop tomorrow. But I think I'll go to Sterlings to get some GOOD blades first.
Again, many thanks.