Thanks muchly. I'll get on that right this second
*Edit*
So. Having done what you said I now have a game map that doesn't give me any more detail than a nautical chart, plus I can't zoom in to see the sub in its compass rose, which is exactly what I wanted. If we were within reach of a pub I'd buy you a beer, but since we're not, my heartfelt thanks will have to do. What I found was, when I deleted the first few zoom levels there was a fuzzy black border to the map that hadn't been there previously. When zoomed in this turned out to be a grid composed of individual minutes, also when I tried ctrl+click it put me on dry land. So instead of deleting the zoom levels I didn't want, I just replaced them with the highest level that I wanted. That worked fine as far as getting rid of the grid at low magnification and centering the sub, but the grid was still there when zoomed in - it seems that I zoom in up to the limit I set, and when I go past that I get the grid that i mentioned, even though the zoom level doesn't change. I'm not too fussed about this as it will be quite useful for plotting DR and AP - On a real chart I'd use the latitude scale, but that doesn't work on this one, as you're no doubt aware.
Apologies if this is too much detail, but I figured since you shared your knowledge, I'd share my results - It's possible that someone might find them useful.
Thanks again