70012 was a separate locomotive in its own right, with its own, albeit curtailed, history; 70099 also has its own identity and history. No asset register should ever conflate them
its also worth remembering that (assuming my memory is working pre coffee) on Tops, you can't re-use a loco, carriage, unit etc number within so many years of it last having been used.. iirc it was 3 years since last use on TOPS.
hence why GBRf have been able to utilise the 73/9 number range for its re-engine class 73s when Merseyrail/Travel had used the 73/9 numbers for its 73s that were originally 73/0s
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Surely TOPS isn't an application matter for TOC's? That surely has to be dealt with by a body such as Network rail or something similar that deals with allocating TOPS/Access charges etc etc? However I digress.
well GBRf have, in the past, applied to the Rolling Stock Library (I think its them), who administer such things, to have their 66s that were previously with DRS, Freightliner or Colas renumbered from 66/4s, 66/5s or 66/8s into 66/7s...
Freightliner and DRS have not done the same with their more recent class 66 aquasitions ie DRS and the 66/3s , and Freightliner and the ex DRS 66/4s.
so it must be down to the operators regarding number changes... unless a loco, carriage or unit number were to conflict with the number of an existing vehicle... in which case I suspect it would be upto the RSL