BAW-198 Fuel Emergency
 

Turkish media reports that BA flight 198 from Mumbai to London while over Bulgaria declared a fuel emergency and landed at Istanbul at 18:05.
At Istanbul refueling took place and afterwards the flight continued to London.The flight was operated by a 777.
Just to clarify this I am not speculating on anything i just re post what I've read, although it looks a bit weird to me. Thanks

Very unlikely they declared a fuel emergency.
More likely is that due strong headwinds/more trackmiles/unfavorable cruise level/..... they were not sure they could reach London with reserves intact and made the tactical decision together with OPS department to make a fuel stop in IST.

Medical emergency according to the DM. Sounds more likely.

British Airways Flight BA198: RECAP after plane from Mumbai to London Heathrow enters holding pattern - Mirror Online

Incident: British Airways B772 near Istanbul on Apr 11th 2015, fuel leak
Avherald reporting it as a suspected fuel leak.

I believe it was indeed a fuel problem as described by AV Herald.
BA Source says G-YMMU diverted to Istanbul due to a technical issue.
G-VIIF was sent to Istanbul to collect the passengers and G-YMMU appears still to be in Istanbul.