Phew.!!....That was some fuel leak There was another Hermes flight that I feel I should mention because it came so close to being a catastropic disaster. We were returning to the UK, empty, after a flight down to Aden. We refuelled at Malta and took off, at night, bound for Blackbushe,UK. After about an hour, as we were running into cloud, we put the wing floodlights on,to check if any icing had started on the leading edges. We then noticed a white plume flowing backwards from the top of the Starboard (right) wing ,and realised that it was fuel. Somehow a fuel cap had been left off and the fuel was syphoning out,although the fuel guages,on that side , showed normal. At first we thought it would stop after the initial surplus had cleared, also as the fuel guages showed normal. Then the skipper remarked that the Port wing seemed to be getting heavier and that he had to wind on some Aileron trim to get it back up.!. However , the skipper decided that something was not quite right and decided to return to Malta, after notifying our company in uk, by radio. As we were too heavy for landing weight we got permission to dump fuel in a prescribed area. As we dumped fuel the Port wing seemed to come up and the Aileron trim reduced, but still the starboard fuel guages showed high.!  We now lined up for landing, but the control tower reminded us that due to some fault in the landing lights they would be using"Goosenecks",which are naked flame lights.!!.( If we had known the extent of the trouble, we may well have thought very carefully about landing!). Anyway, we continued the approach, and just as we flared out to touchdown, the starboard fuel guages dropped to zero.(no more lift). We taxied round to the dispersal point, where they had recovered the fuel cap, and started to refuel. As they started putting fuel in the outer Starboard tank, we were absolutely shattered to see fuel coming out of every crack and rivet hole under the wing.!!, and stopped fuelling immediately. Well now......apparantly what had happened is that the fuel tanks were not of the rigid type,as you might imagine, but were flexible, and known as "Bag tanks". As the syphoning had started, it began to draw the tank slowly upwards, in turn lifting the fuel guage floats (hence the higher readings) until some point was reached whereby the tank was starting to be torn and the fuel flooding into the wing area.!!. Someone up above must have been watching over us!!!.