The relevant part of the BBC interview is:
"I met Tom Baker doing a voice-over when David Tennant wasn't at all well known. We were doing this voice-over together and I said to Tom, 'Oh, my friend's a really, really big Doctor Who fan,' and he replied, 'Wait!' He got his cheque book out and asked, 'What his name?' I said 'David Tennant'. He wrote, 'To David Tennant, seventeen pounds forty five', signed it and I asked him what it meant. He said, 'He'll know...'"
As a postscript to this story, David was asked in The Nerdist what it meant and he said he didn't know!
Some fans think it may have been the time which some of Tom Baker's episodes of Doctor Who were broadcast.