David Radcliffe's 'Modern Private Owner Wagons on British Rail' gives the TOPS numbers of the Esso 45t class A tanks as 56000 to 56281 which concurs with Paul Bartlett. 6000-6199 is only the Pickering built batch, there were also Chas Roberts (6200 to 6240) and Powell Duffryn (6241 to 6281) batches.
Edit: Having Googled images of the Bachmann wagons, I see that they are BRT branded which means that they are not the ESSO owned batches but hired in. Take a look at the first photo in this gallery:
Edited by giz, 14 January 2018 - 19:49 .