What is the best method of cutting kitchen worktop to give a nice
straight edge? I have a hand held circular saw and a jigsaw, the
circular has a bit of an aggressive blade however.

The circular saw out of those two. It is not really possible to make a good
cut with a jigsaw, although they are fine for rough work that isn't visible,
such as sink cutouts.

For really decent cut, though, you should be looking at a 1/2" router,
having cut it about 3mm oversize with either jigsaw or circular.

Christian.