I thought I'd close this one off with what happened incase anyone else ran into this.
I opened a support case with Apple and it turned out to be caused by:
Our engineering teams found that the transfer caused duplicate App IDs
I was asked which App ID I wanted deleted and after I picked one, carefully, the transfer went through.
Don't know how I managed to created duplicate App ID's, it's an old App, but I had and that was the sticking point.
Transfer was completed successfully. All good now.
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect
Tags: