Whilst the fault has probably been found by now, the date and flight number with your observations about the high speed of the oscillation could be written or faxed (a phone call really won't get far as they will probably require a fair amount of explanation):
London Heathrow Airport Office
Room 6204
Departures Mezzanine
Terminal 4
Heathrow Airport
TW6 3FB
Fax: +44 0161 838 5398
Or try the Feedback side of their website but I think that will take ages to get to the technical department.