A surprising number of people on banknotes haven't been in DW episodes

Wales Online fills in such details as there are: "BBC accountant who controlled programme budgets for Doctor Who and Casualty has been jailed for two years after pleading guilty to 17 charges of fraud amounting to £80,000. Oliver Ager, 35, was said at Cardiff Crown Court to have committed the offences “in abuse of his position” with BBC Wales. Ager, of Rock Avenue, Nailsea, Somerset, was said to have been dismissed from his job and to have paid back £30,000 of the money from his pension fund. There was no order made in respect of the remaining sum. He will serve half the two-year term in prison before being released on licence."

It's not entirely clear what happened - apparently some allegations were made and an internal investigation started and found to be accurate. The BBC doesn't wish to provide further details, understandably, so there's no knowing if the money went missing from the Casualty budget or the Doctor Who one (he worked on twenty episodes from the middle of Series Three to "The Next Doctor",  as per his IMDB page, and then moved to Casualty). There was one official comment: “Since Oliver Ager committed these crimes the BBC has reviewed its cash processes and ensured that there is additional oversight to prevent a crime like this from happening again.”

The Wales article says that he was "director of an independent production company called Different Productions Ltd from December 2011 until October 2012, the month he was dismissed by the BBC." Which there aren't any records of online, barring a unit in Buckinghamshire, so it probably wasn't a large-scale proposition.