Posting for anyone else who might find the information useful.
After yesterdays upgrade I could access the site fine apart from not being able to post because there was a blank space where the text editor box should be. It worked using Chrome but I don't like Chrome so wanted to resolve Safari.
My MacAir is quite old now (2013) and was running on Sierra which is clearly too old to support new Mumsnet - I've also had some niggles on other sites so might have solved that too.
Because of it's age auto updates stopped being supported some time ago but with some faffing you can upgrade to a newer MacOS. After researching I went with Mojave because I figured it's new enough to handle MN but not too new my old Mac won't handle it or lack the storage space.
Macs of a similar age to mine can technically support up to the Catalina OS but I was warned off that one because unlike previous OS upgrades it quite radically changes hardware.
This is a guide on how to find the best update
support.apple.com/en-gb/HT211683
It seems it's common to run into problems with older installers but I found the following fix which worked for me.
apple.stackexchange.com/questions/340345/damaged-macos-mojave-installer
As per this discussion on Apple support community, you should be able to overcome the issue by deleting InstallInfo.plist file from within the macOS Mojave installer app.
Go to your applications folder.
Find the installer for macOS Mojave.
Right click on the installer and click "Show Package Contents".
Click on folder named "Contents".
Click on the folder named "SharedSupport".
Delete the file named "InstallInfo.plist".
Enter your administer password to confirm.
Now open the installer.
I'm no expert at tech stuff but none of this was beyond my capabilities. Hopefully it might help anyone else struggling with the same issue on an older MacBook.