Thanks, Francois. I'll be sure to bear that in mind when I do my next set of exposure calculatio....heck, who am I kidding?!!
Well, given the summer we're having I would need to move somewhere which does a better job of being sunny than the UK if I decide I want to use this process seriously.
Update: This afternoon I tried a 3+ hour exposure in full daylight, but with very little bright sunlight and got zippo, nada, nichts, rien. It looks like bright sunlight is the key. Tomorrow, I'll point it out of the upstairs window and leave it there all day....I should get a result of sorts.