Thanks to Mark 1 for his comments. firstly, I want to correct the obvious error in my original post - the R4 tx is of course at Droitwich, not Daventry! and pushes out 500kW these days... The only activity at Rugby now is Loran C, the time signal is now at Anthorn in Cumbria. However as both are VLF, I don't think that they would affect the satellites which are around 1600 MHz?
But I'm sure Mark 1's comment is valid, as the signal is very weak to a GPS. The reason for starting the thread in the 1st place was twofold, one just curiosity as to what's causing the problem and two, to remind people of the vulnerability of hand held GPSs, which seems to be rather more of a problem now than when I used one regularly.