There is definitely a "problem" outside of London and the K&A, relating to the abuse of visitor moorings. Rather than put an exact number of boats on it, I'd suggest a better way of expressing it is to express the % of space on a visitor mooring taken up by overstayers. What typically happens is there is a proportion of overstayers, a proportion of boats within the (restricted) time and a proportion of free spaces. Over a given timeframe, a handful of overstayers will eventually move on but many more short term stayers will move on too, some of these 'fluid' spaces are taken up by other short-term stayers and some are taken up by overstayers (and a few may remain free).
Eventually, the rate of incoming overstayers exceeds the rate of leavers, thus the visitor mooring becomes more and more full, resulting in fewer opportunities for short term stayers. First, the free spaces will go and then it becomes "one out one in" and these will be gradually taken by overstayers. The existing overstayers will notice the pattern of behaviour and "stay put" (after all, if they've overstayed 3 weeks, or 6 weeks, what's the difference?)
In due course, enforcement action might occur (and a big proportion of overstayers will leave.....), or it might not occur and eventually the VM is full and ceases to function as a short term mooring spot.
This means the genuine CCers and those cruising, probably time-limited, for a short break (ie home moorers on a holiday/cruise, or hire boaters) unable to use the mooring and have to go elsewhere.
Is this fair?