Someone asked about these a week or two ago, and I have just come across something that might be of interest. I aquired a copy of The Railway Observer Magazine for October 1947 and it has the following comment.
Hither Green: Considerable changes have taken place since 1939 in Locomotive workings on through trains from companies north of the Thames. there is now no GWR working and so Hither Green loses the distinction of being one of the few places in the country where one could see engines from all four groups together. It then goes on to list the sheds and common engine types for the
1947 working.Pre 1939 would also be interesting with the GWR in attendance as well, a dream location for someone who likes all kinds of Locomotives and wants a reason to run them side by side.