I have a cheap circular saw from B&Q with a 20mm spindle and, I
think, 160mm blade. I want to get a thin blade for the saw to use to
cut access hatches in chipboard floors. Whenever I Google on the
subject clearly loads of references to products come up, but they
don't mention the width of cut. I did find a "fine" blade on the
Axminster site, but wasn't sure if that referred to the pitch or cut
width. Perhaps the 2 are related?

I was thinking of setting the saw to 45 degrees and just (!) deeper
than the board depth, then cutting a rectangular hatch.

What blade should I ask for and is this idea going to work anyway?

Thanks,
BraileTrail