Its very difficult to respond sensibly to Tyke's rather primitive evaluation of the Earth's Magnetic Field. The fact is that , as Ralphie says, it is too weak. Don't worry about it. The strength of the Earth's magnetic field is far too weak ...about 0.5 Gauss...to do much. A typical magnet capable of serious damage to magnetic disks or tapes requires several hundred Gauss at very close range.

The problem here is that the "economy" driven Bulmer might store the tapes in close proximity to another kind of damaging source - the loudspeaker, for instance. I doubt if he has even the basic disciplines required to create a proper tape store - ie one that is substantially free of fluctuations of light, heat, cold and humidity, not to mention odd bits of old iron that we are led to believe he collects routinely.