Recommended reading if you can find a copy, SILENT VICTORY, which covers all the screwups in the first two years. Particularly critical of Christie and bureau of ordinance for ignoring all the evidence the Brits had learned the hard way, really no excuse for refusing to deactivate something that our far more experienced allies had already proven worked less than 5% of the time and only in perfect conditions.
Some other reading;
http://www.ngdc.noaa.gov/geomag/declination.shtml
http://www.uboat.net/allies/technical/mad.htm
When I was on active duty (late 70s early 80s) MAD was more sophisticated and effective than the WWII stuff, but we still had to deal with all the quirks of the earth's magnetic field and double check everything with sonar.