Hi,
It was over 25 years ago so I can only guess.
According to my OS maps Stansted is just over 50 miles away in a direct line from Gatwick and IIRC they worked distances based on SID/STARS and for that I believe it was in the order of 75-100nms. Not sure how you measured it.
Head Office (Oakland) used to make the final decision on alternates based on the information we supplied from the area offices. In those days if the forecast was good they would often work very much on minimum fuel loads.
As stated in my original message when the a/c first arrived in the area the altn wx was fine, we diverted the a/c to Stansted due to Gatwicks WX and they were holding for Stansted due to congestion when the Stansted WX unexpectly dropped below that suitable for an altn and although I can't recall for certain maybe even as a destination airfield. All this combined to eat into the fuel reserves at the beginning.
While WX often caused problems in my 8 yrs in ops, this time it caught all three of us, from ops manager downwards, totally by surprise. Going on the messages on the ops freq which we shared with others we were not the only ones, on the ground or in the air.
In my next, and still current, job as a police controller, I was coming to the end of my first week of night shifts when a certain hurricane wrecked the south coast. To say we were busy was an understatement but I remember thinking I would rather remain were I was rather than in the job I had been three months before.