After ruining lots of costly materials with the ruler slipping mid cut I searched the odds & soda box for a solution I thought of a pan brake without the bend facility.
I came up with a bit of 6mm x 9mm carbon steel bar with 6mm holes about 300mm apart a piece of 19mm chip board 2 x 1/4w bolts & 2 captive nuts to suit.
After some time the knife marks in the chip board became rather deep so I cut an old cutting mat in half it has worked well for several years.
After using the holes drilled in steel as a quide for holes in the base board the back wes recessed to take the captive nuts.
With a card writhing book cover to show assemlbled for use.
Not pretty but functions well, in hind sight 1/2" steel angle would be better than the hardened steel I used as it tends to lift in the middle if over tightened.
I hope this will be useful to some fellow modellers.