Hiya bigflesh, was this the beastie you saw?:
http://www.airliners.net/open.file/1025320/M/
If it was, I think it transports Airbus A300 parts around Europe. I think it takes wings from near Manchester (I think it's Brough) to Hamburg where the Germans build other bits and eventually the whole thing gets erected in Toulouse. Terribly complicated, but it gives the countries who financed the construction a part in it. I've never seen one of these myself- i used to see the Boeing Stratocruiser with a fat fuselage they used long ago. If over Sheffield, it would be fairly low- i don't think these guppies fly very high anyway. 60,000' is about 10+ miles high, and only Concorde and the SR71 Blackbird (and other military fighters) go that high (that we know about). Commercial jets cruise about 31,000'-39,000' usually, but it can take 60-100 miles to climb up there (and get down again).
Watch out for the high things with a delta wing. I think it's called the Aurora and it's still top secret! What you need is some binoculars, just cheap ones will do, not too strong or you'll never be able to find the aeroplanes. Have you got some?