I am running Thunderbird on a new MacBook pro. In settings I have indicated that the write window is to obtain contacts from my 'synced contacts' however it ignores this source.

Can you be more precise concerning the settings where you indicated those "synced contacts" (compare with my image)? Do you use the Mac "Contacts" and iCloud ?

Thanks for your thoughts. I use Thunderbird on a Windows PC as well so cannot rely on Mac or iCloud contacts. My synced contacts are stored on a NextCloud server. I am not sure how to enter the parameters of this service into the 'edit directories' box.

I think that you should open the "Address Book" tab in Thunderbird on your MBP, then go to File > New > Address Book > CardDav Address Book ... which will open open the Account Hub for a new remote address book. Unfortunately I am not familiar with NextCloud, so I cannot help you to fill in the URL/Hostname of your remote address book (see image)