UA28 FR24
 

Not sure what UA 28 is doing south west of the Isle of Wight but has been doing it for an awful long time, even for a fuel dump?

Not sure but he advised the controller that he would be holding for approx 2 hours before returning to LHR. Was just burning fuel, now dumping.

The Newark arrivals board has the flight posted as 'Diverted'

UA28 FR24
 

It's been holding for an eternity. Even with a tech issue requiring a fuel dump and return to LHR it should be back long ago?

From the BBC radio news, returning to Heathrow due to a 'maintenance issue'. Unusual that a pretty normal and oft experienced happening should make a mention on the news ?

UA28 FR24
 

Agreed. I don't know the 767 that well but unless it has an electrical bus issue that is also rendering the fuel jettison system inop so he's having to burn it? A normal airborne return doesn't take over three hours when so close to the origin?

It's most likely as you say, needs to burn fuel the old way, so they're at 12,000ft for a few hours and then RTB. From the beeb….

13:30: Flight returns to Heathrow 'over maintenance issue' A US-bound United Airlines flight is returning to London's Heathrow airport following a maintenance issue, the airline has said.
Flight UA28 to Newark, New Jersey, had been circling off the south coast of England for about two hours.
It has 227 customers and a crew of 13 on board.

UAL 28 Dumping Fuel
 

UAL 28 Dumping Fuel south west Isle of white 12000ft

UAL 28 returning to EGLL
 

United flight 28 LHR-EWR, Boeing 767, currently holding SW of Isle of Wight,FL120,airborne for 3 hours,as of 1509GMT.

Leaving the hold now inbound LHR

UAL28 LHR > EWR
 

been circling for 3hrs plus, did see a brief mention on Twitter that it is maintenance related but can't find substantiation

have deleted image due to it's size will edit and repost

it has just broken out of hold pattern and appears to be heading back to LHR

No image of hold it remains too big for page

From Reuters: UA28 Returning due to "Maintenance Issue"

U.S.-bound United Airlines flight returns to London after 'maintenance issue' | Reuters

Now out of the hold and heading 040.

15;39 local, he is now descending from FL080 for LHR. After over a dozen orbits in the Isle of Wight region.

Landed 15.45 GMT LHR

good news - appears to have landed safely.

As of 15.53 on the ground tootling along towards T3 at LHR. Glad that it wasn't too serious, but am curious as to what it was :8

Daily Mirror has a tweet from a passenger, who took a photo from the apron while disembarking, of left hand engine with cowl open and being looked at. May be a clue.