Thanks all for your input so far, much appreciated.
Just to clarify a few things - we are both connecting with a wireless connection.
I am seeing download speeds around 25-30Mbps which should be more than sufficient.
It alternates which one of us is having issues, today I couldn't even leave the tavern after logging in whilst she has zero issues, yesterday I was fine and she was jumping all over the place.
We were both playing together absolutely fine until last Thursday and have had 4 different occasions where we have experienced this issue.
Have restarted the router multiple times, restarted PCs, with both of us inviting to game to no effect.
The router is less than 6 months old and performs with zero issues on any other application.
No settings have changed between having no issues and being completely unable to play together.
I have been playing just fine in Galleon groups with other players from all over the world for hours on end on a daily basis, issues only arise when we both join a game together.
I agree that it has the hallmarks of a router issue and I'll see if I can find an RJ45 to test a wired connection, but this isn't a viable long term solution and as the connection seems to be without issues anywhere else I don't really want to shell out on new hardware that shouldn't be needed.