TCTC,
If you were flight crew using 121.6 in anger, then file an ASR.
If you were listening in, then bear in mind that as HD says it's illegal, and also bear in mind that most airports use 121.6 in the UK for airport RFFS. It's use is restricted to operations on the ground, and the transmitters in the fire vehicles will be very low powered, so as to ensure using 121.6 at LHR doesn't interfere with using 121.6 at LGW or Farnborough.
If you're listening in from the ground some distance from LHR, this may explain it.