Any idea why its circling over Southampton ?

Should be in the spotters section really.

Not squawking 7700 so I would suggest it probably has some sort of technical problem and is burning fuel (presumably US Air 330s can't dump fuel?) before most likely returning to Heathrow. Bit of a non-event really.

Edit: in fact it's heading back towards London as I type...

Seemingly on the ground back at EGLL now.

Jumbo jet circles Southampton after undercarriage problems (From Daily Echo) (http://www.dailyecho.co.uk/news/district/southampton/9689883.Jumbo_jet_circles_Southampton_after_undercarriage_problems/)