Interesting to note the two bits of ground equipment in the last pic of G-OATP were originally Britannia equipment which transferred to Servisair when BY ceased self-handling.
The tug was obviously used for 767's in its early days (looks a bit over-sized on an ATP!) The other bit of kit was a mobile ground power unit. The drive came from the rather beefy generator motor (which was limited!) However, it was easy to by-pass the limiter which turned it into a lethal projectile with useless brakes. I recall the thing ended up modifying the wall of the newly built Line office outside H89 a long time ago!.....