I would suggest that the aircraft was approximately 100 – 150 feet above us
I very very very much doubt it...ATC certainly wouldn't allow that and even if they stuff up TCAS (the collision avoidance system on the Flight Deck ) would also be demanding 1000' separation - and demanding a fairly healthy avoidance manoeuver if that distance was going to infringed that seriously.
As C.V. has stated it's 1000' vertical separation for airline traffic at cruising levels over most (all?) of Europe these days, which can look very close, even to those of us with lots of years up front, civil and/or military
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There were audible gasps from many of the passengers that witnessed the event
Add in a high closure rate, and perhaps the element of surprise and it can certainly produce a "startle" factor!!!