Sounds like the incident I spoke to one of the reporting pilots about (he's still serving so I shan't mention his name). According to him, it was below and quite close, so not debris on re-entry, had windows and a blue pulsing 'doughnut' exhaust.
Curiously, he noted the rough heading of it and a few days later came across a newspaper report that a BA pilot had reported something very similar. He actually traced this guy, went to see him to check out his story and it did....they reckoned it was the same thing, 15 minutes apart, same description, same heading. Trouble is, his sighting was over the North Sea, BA pilot was over Italy! In 15 minutes.
This is what he told me. I cannot vouch for its accuracy, but I know him and he a pretty straight guy.