Regional Guy,
If a trainee pilot, i.e pre license issue lost their logbook, then it should be a simple matter of copying the training records into a new book and getting this signed off by the appropriate training school.
Should a license holder lost their logbook, then the flights post license issue could be accounted in the new log book but would need to be accompanied by an afidavit drawn up by a solicitor and signed by the license holder. The CAA will then look at this record and make a judgement as to what they will allow to be credited, bearing in mind they would have seen the original logbook at license issue and have a trend established.
If a commercial pilot loses his/her logbook, then a similar afidavit would be drawn up, but the records are more auditable since all AOC holders have to keep copies of flight duty hours on a monthly basis and some checks would be made.
Should any Pilot just turn up with a new logbook stating a number of unsubstanciated hours, then this would be considered with extreme caution and proberbly not accepted as a true record.
Most sensible Pilots photocopy their logbook on a regular basis just to cover these situations.