In reply to TrekGib:
Wheel wobble. Not uncommon. I think the forums on road.cc had a lot on it in the past week or so. Various theories as to what causes it/what it is, but if you take your front wheel off, spin it and then twist it, you can get a very similar effect (gyroscopic wobble) which to my mind is very similar. To stop it, ease off your grip on the bars and rest one or both knees on the top tube and grip it. If that doesn't work, one foot down, one knee on the top tube and force a twist into the frame, with a bit of back brake too. Keep your weight in the saddle, else it will all go wrong quickly.