This is the clincher. the second photo almost certainly shows 80098 as the daubed paintwork looks the same as on my photo. Other posters have 'proved' that 4920 stayed with the same Collett tender. The two tanks and the Hall appeared to stay together the whole time, even though they swapped positions. For me it's 'job done', so thank you Robin Edwards.I think the clue lies in whether we think this is the same Collett 4000g tender in both images - I think it is and can't find any other with matching weather pattern.
Take a look also at this image of 80080 then what I believe is 80098 and the same Hall tender behind - taken in 1975
Get ready for my next photo!