I have managed Horning Staithe many a time, both in and out of season, however as popular as it is it really is one of those blink and you'll miss it opportunity type of moorings.
As Bluebell says above, you normally have to go for a tighter than you'd care to try spaces but it's all good fun fitting into a super tight spot and enjoying your 10 pints in the local pubs with a smug sense of satisfaction