Hi Mark, I have been out for a while today and have only just had chance to reply sorry about that.
I use FREEOLA for my web hosting. They have upgraded some of the servers they use over the last few days and there were some settings (at their end) that were incorrect for my site according to them.
The site that had a problem is now functioning 100%. Sadly I am on the phone to them again now as I have just found out another one of my WordPress sites is not working properly I keep getting the dreaded error 500.
The website that I posted about in the first place (which is now working 100% spot on) has the latest version of WordPress and Jetpack so it seems both the latest versions of WordPress and Jetpack are actually working fine.
I hope you manage to get yours up and running very soon.
Regards,
Roger