Just open the campaign_SCR.mis and zoom in on the Channel. Here's what I found while looking at Loch Ewe;
http://img131.imageshack.us/img131/4669/ewe5st.jpg,
You can see the minefields and subnets.
SH3Gen is available at
http://www.global-explorer.de/. This will show the approximate location of some minefields, but not their exact postion or size.