I'm delighted to report that someone (thank you Red Orc) has found a sheet of the elusive mapping paper and sent me a scan. As a couple of people suggested the paper is printed with two grids (the second being offset by about 1mm). This means the small gaps between grid lines are ideal for shading in a walls.
I've printed a couple of sheets of the paper out and will try drawing a map on them tomorrow. The main squares are around 3mm which is quite small. I think if I were to print a pad of these sheets I'd want them to be double-sided with a different sized grid on the reverse side.
If anyone would like a copy of the full-size scan, send me an email (oubliettemagazineATyahoo.co.uk) and I'll be happy to send it to you.