WW2 Logbooks are hugely valuable
Whilst that is certainly true in the case of a Dambuster crew member, it is not always the case.
I once went to look at a Mosquito logbook which was being offered for sale by the deceased original owner's family. I had high hopes of some very interesting reading but I discovered that the chap had spent his whole time in Coastal Command, and his ops were recorded as page after page of "Patrol, area X," or whatever. The flying was dangerous and an essential part of the anti submarine campaign, but the unpalatable truth was that it made uninteresting reading. In the event I decided not to make an offer, and suggested that they kept it in the family for posterity.
Hope this chap gets his just deserts. Maybe an offer of a reduced sentence if he produces the logbook might be effective in bringing it to light.