Thanks guys, I think I'll need to dust off the circular saw. My problem was securing a relatively long and thin piece of wood for the saw to bit but not vibrate. I like the idea of an oversized block, though still not easy to secure on a B&D Workmate, even with clamps.
Screw it to the bench (Workmate). It will be rock solid then. 
What you need to do is clamp the circular saw upside down in the work mate, this than turns it into a table saw.
Clamp a fence to the workmate to give you sometinng to work against, set the blade at 45 degs, then you can rip down any size of timber you fnacey.
Whatever you do make sure the fence doesnot run past the centre of the blade otherwise there is a risk of binding on the back cut and throwing the wood straight back at you, which without doubt will hit you square in the knackers!!
Also make sure the circular saw has the writhing knife fitted.
If your feeling flush you can buy one of these :-
Saw benchOr one oth these if you own a router :-
Router Table