it happened at the of end 1976 when i was flying a Britannia 312F from Gatwick to Baghdad. We had been cleared from FL 210 to FL 250 about 30 minutes before reaching Zagreb. It was dark, but the radar was painting a very large CB above and just to the right of our track. There was no turbulence when it started to rain and the temperature was indicating way below zero. Over the next five or ten minutes we suffered a continuous string of engine failures, normally only two at any one time, but we were not able to maintain FL 250, so we were forced to descend! Zagreb would not clear us lower, but we could not comply! By this time we had lost all our airspeed indications, only one altimeter was working, but as it was dark and I had no airspeed readouts I decided to continue towards Baghdad using power settings to maintain a near average airspeed as possible.
The sun came up shortly before TOD. Baghdad cleared us direct to the airport and FL 50. We started a very careful descent, planning a landing without any airspeed readout, but around 10000 feet both airspeed indicators came alive and we made a normal landing. It was the longest most worrying night in my flying career!