Well, things all had a happy ending (no, not that kind…), so thought I would update.
Motorway were the original focus as they were offering the highest price, but had a wobble about the MOT mileage discrepancy.
In the meantime, we went down the WBAC route as they had a branch close to Mum in Greenock. They offered damn near what Motorway did, no qualms about the mileage history as they indeed were happy with the Merc digital service record. Deal was done very quickly and we were all happy, mum with a nice wedge of cash which helps to soften the impact of how many bills she has to deal with after dad’s death (and the wind up of the business).
For both mum and I, this was the easiest and fastest route, and it worked out well.
Today, I get an email from the DVSA saying that they cannot change the mileage historically, but that they can change that measure to KM as that was what was recorded in error. Huh, go figure.
Alls well that ends well, thanks all.