The rear diff was intended to take Castrol TAF-X which hasn't been available for some time.
I emailed Castrol tech team asking about the current replacement and within the hour (on a Saturday!) they said.....
Hello Justin,
Thanks for contacting us.
The product who replaces TAF-X is Castrol Transmax Manual Transaxle 75W-90
Please do not hesitate to contact us for further questions.
We hope we have been able to help you.
Kind regards,
Castrol Technical Service
I found Amazon had sellers able to offer a good price and next day delivery but make sure when you buy that you are getting the right product as it should look like the below with the words "Transaxle" in the grey box as there seem to be slightly different versions of this
Opie Oils of course always stock it but not sure our discount code works still..
https://www.opieoils.co.uk/p-6363-castr ... fluid.aspx