The Creamery siding at Bailey Gate (the one where the milk tankers were handled) isn't shown on any of the online signalling diagram maps of the yard. It ran at the top of the map from left to right (roughly parallel with the main line) and was fed by what is called 'Down Siding No 2'. The 'fork' in the track layout at the top of the 1955 map didn't exist but rather the track continued the curve left and fed the Creamery siding joining it from the right hand track of a right handed point positioned roughly half way along the Creamery siding's length. The Creamery siding therefore had a 'kick back' section running to the right that tankers could not be shunted into directly, but which all tankers had to be pushed into.
That's the best way I can describe it, appologies if my description is confusing, or if I'm preaching to anyone that knows the layout already.
Tankers always arrived from and departed in the 'up' direction, ie from/to the left on the map. They were usually marshalled at the front of an arriving train and the rear of a departing train. An arriving loco would have uncoupled from it's train leaving it in the down platform, run forward, and then shunted back along Down Siding No 2 to the Creamery Siding, and pulled out any departing tankers, shunting them into the siding immediately behind the down platform for collection by a later up train. Having emptied this half of the Creamery Siding, the loco would then shunt back to its train still in the down platform, couple to the tankers (which would now be uncoupled from the rest of the train), haul them forward, then propell them back into the now emptied left hand (north west) end of the Creamery Siding.
The loco would therefore be at the wrong side of the tankers to be able to shunt them into the right hand (kick back) end of the Creamery Siding. It would need to be able to run round them somewhere. It couldn't do this on the main line because the rest of it's train was still in the down platform and blocking any 'run round' (this could be a passenger or goods train). The only other place to run round, was via what is marked on the map as 'Unigate Creameries Siding' (not to be confused with what I am calling the Creamery Siding) which was the new siding loop added to the yard in 1955. However, this was next to a loading bank where photographs show vans usually stabled being loaded with ????? (whatever), so it would be blocked and couldn't be used for running round .
This is why I believe using a loco to do all the milk tanker shunting movements isn't practical. Moving them manually was presumably the most convienient way for the dairy because they could move them from/to the arrivals/departure end of the siding when they were ready and not have to rely on a convienient loco.
Appologies if my explanation/reasoning is a bit confusing, this is quite difficult to explain without having the map here.
I did wonder about motorising one tanker as an 'interesting exercise', I might still try it. And the magnet under the baseboard idea had also occurred to me too. Every tanker would need to be fitted with its 'lump of iron' between the frames, but this might be the best 'simplist' way of avoiding the 'big finger'.