Something else modern railways do which is a necessary side-effect of the way they present the data, is divide the total fleet mileage by 1 if there were 0 TINs in the period. I took a look at the last few months back until I found a TIN - I think June had three, so I summed the June/July/August figures and divided by June's 3 TINs, it gives a figure of around 90,000, which over a 3-month span is enough to put them certainly in the top 10, probably the top 5 overall.
Edit:
707s had 3 TINs in July, so July + August's mileage / those 3 = 127333 MTIN
That would put them 2nd place behind last year's leader, the 350/3 at 138,579.
2017 saw both the 450/444s exceeding 90,000 as well but post-SWR they have fallen to the middle of the pack. Whether that's due to questionable accounting on SWT's part of SWR's lesser maintenance regime, I suspect it's both.