The message
from "Sally B" sb@(no spam)trefonen.eclipse.co.uk contains these words:

I am trying to cut 40mm thick Maple worktop with an oldish Bosch 450w
jigsaw
and new Bosch bimetal 'clean for hardwood' T101BRF blade, but the saw keeps
kicking/jumping off the work, despite my holding it down firmly.

You'll never manage it properly with a jigsaw. Use a circular saw. Screw
a batten to the reverse side (use short screws!) to give you a super
straight edge.
Once the batten's there you can use a router instead if you have one
meaty enough - but not a jigsaw.

--
Skipweasel
Pay no attention to that man behind the curtain.