Further info, if i change the comment display order in Settings> Discussion > newest first then the problem persists but instead of the comments showing for the default (newest) it shows once I select 'oldest. 

It's actually the same problem I was having here that has crept up again. There is no caching involved as it's in Learndash. Would you be able to make this private so I can share my creds and you could take a look? thank you.

Many thanks this has worked.