Sigh indeed. I got home on friday and realised I'd left the pack of MGiv paper open. Tested one and there's a strip of black down one edge, the one from the top of the stack is probably a goner.
so, more updates. The 8x10 I shot friday turned out very very black. But not so black from a light leak, there were a few spots of white from some deep shadows. Scanning the hell out of it I got some kind of image, but I definitely need to rate it a stop or two faster than the ei3 when I run with no filter.

Today I took it out again, and this time I found a B+W 52E 2x Yellow-Green filter to put on it. The clouds are still around, but the sun was trying to break through a bit. EV10 at ei6, minus the 7 stops for aperture gave me 2 mins so I went with that (ie, it was ei6 including filter factor, or ei12 minus one stop for the filter).
I got something almost repectable, a little thin and the shadows look totally white, but better than all black. So maybe ei3 including filter factor for the 52E might be the way to go.

I've also bought a different step ring so I can mount a 39mm real Yellow filter on it. I've just looked to see what this one is and all it says is "Ernst Leitz GmbH Wetzlar" and "1" and "Germany" on it. I've never noticed that it doesn't have a filter factor printed on it because it lives on my TTL Bessa. So once I try this one I'll have to experiment again.