Thanks for the responses! The linked article above was particularly interesting as although logically some sort of system must operate, I wasn't aware of the details. It's all very reassuring!
I must say though, it did feel like something was a little awry in this case, and I'm no 100% sure that what I saw fits into the scenarios in that link. The aircraft was certainly not several miles away - 2 or 3 at the absolute most. It is concievable I suppose that it could have been at a different height, but it was absolutely at my eye level and given the proximity I think 1000 feet difference in altitude would have been noticeable - and as it drew closer there was no percievable change in our vertical seperation.
Anyway, I'll take your words for it that it was a normal occurence that just looked odd to someone who doesn't fly accross the Atlantic very often!!