Flappy lumpy chain and possible leaking gearbox output shaft

[Gnasher]

while i was in there the output shaft looks like it might have a wee leak. Do I just pop the gasket and drift in a new one?  thanks in advance.

There isn't a gasket on the output shaft but rather an oil seal that can be removed and replaced.  Before you do go down that route give the sprocket area another really good clean this time take off the front sprocket really get every trace of the old spray on lub off.  Another really annoying trait this stuff has is that once cleaned it softens and turns back to oil and over a period of days it starts to run giving the appearance of an oil leak.  It's even worse when owners fit a Scott oiler the stuff just keep running and running and running for months and months.  :wall    Get EVERY bit of it off it can take quite a while off scraping, rubbing and jet washing but it's worth it, don't aim the jet wash directly at the output shaft oil seal.  Once thoroughly cleaned you will then be able to see if the oil seal is leaking or whether it's just the old chain lub running.  Then fit a Scott oiler