I'm in the process of clearing loft space for storage. The old water
tank is being chopped up and removed. The existing floor boards are
sparadic, some hang over half way between joists. So I intend to cut
some of them back to the joists in a manner that I can then lay the
chipboard tongue and groove stuff.

I have so far cut the floor boards flush with the joists using my
jigsaw. I now need to cut the floor boards back to half way across the
joist leaving room to lay the new chipboards.

The question I have is what is the best tool to do the job? I've
borrowed a circular saw but it seems a bit like a sledge hammer to
crack a nut.

I am a DIY numpty and I'm not particularly experienced with what tools
might be available and for what use. Any help appreciated!