Bit of a dilemma if one's logbook has found its way into the wrong hands and suffered damage such as indelible graffiti...
Assuming the writing is still legible, get another logbook. Tally up the totals in your old logbook and transport these into the new logbook as the first line, or something like that. Put your signature next to it and a statement "hours flown from <date> to <date> - see logbook #1".
In the very odd case that the CAA or somebody else will want to see your logbook #1, send it along, together with a cover letter explaining what happened. But most likely you'll find for most, if not all of the experience you need to prove in the future, it'll be contained in logbook #2 anyway.
You don't have to restrict yourself to using just the space on one line - if you want to enter a lot of useful stuff after a particular flight just use the next line.
Agree. Use as much space as you need and just record the next flight on whatever happens to be the next empty line. Although there's a point where things do become plain silly: If you need a full page to record the particulars of each and every flight, you've probably gone overboard a bit.