Thanks for the reply, we don't have any support or access from Adva as it's supplied by the carrier. This circuit is also being terminated at the end of next month (the entire reason I've been using it to test, as we've already moved traffic away from it).
I've done further tests, I put a Mikrotik between the TNSR box and the Adva and I can verify via a pcap that the traffic is being transmitted out of TNSR, I can verify it's also leaving the Mikrotik to the Adva - and in Wireshark it looks in no way obviously different to the none dropped pings. Where it gets odd is the Mikrotik now gets packet loss to the other side of the Adva. It didn't used to, so I am thinking the issue is very much on the adva/carrier side and not the tnsr side. Just it only manifested itself when we started to test tnsr. So we jumped to the conclusion that it was an issue in tnsr.
I think in this situation I'm going to have to ask the carrier to investigate.