Thanks Doug,
Of course the handle is relevent, I have two, both with chisel blades, an X-ACTO, with a 1/2'' blade, (thanks for the telephone no.) but also an american one with what looks like MAXX on the handle this had a blade that was about 5mm, I say about, because this afternoon it snapped, and has pinged off somewhere in my workshop.
The decking planks are only 1.5mm wide. the 1/2'' blade is a bit cumbersome