The Directory Service App Log gave lots of errors talking about "The Knowledge Consistency Checker (KCC) being unable to form complete spanning tree network topologies" but nothing really useful to work with.
I first used repadmin /replsummary to try to see what was wrong, but it only confirmed that there were errors.
I then ran DCDiag
On Starting test Replications it said: [Replications Check, Replications Check] Inbound replication is disabled.
To correct, run "repadmin /options
Also Outbound replication is disabled
To correct, run "repadmin /options -DISABLE_OUTBOUND_REPL"
Now on the face of those commands it looks like its disabling the inbound and outbound connections, BUT ITS NOT!
What it does it toggles the current setting to the opposite of what it is already, so if its disabled it enables it and vice versa.
Forcing some replication and running DCDiag shows that this has resolved the problem, although why it occured I still dont know - any ideas anyone ?
Thanks to the pointer in the blog The source server is currently rejecting replication requests