Have used the 27XX series for 100s of customers with SIP trunks + PBX, but since starting to use the 2766 we've had serious issues with delays in calls connecting, delay in audio and sometimes complete calls failure. Anyone experienced anything similar? Upgraded to latest UK firmware.

There's another post about it on this sub-forum that's very similar to your issue, could you make a support ticket for it? That way, the support team can investigate and resolve the issue.

You could try turning off the Auto SIP QoS under [Bandwidth Management] > [Quality of Service] but I'm not sure that's the root cause of it.
The SD-WAN features on the Vigor 2865/66 / 2927 have some VoIP quality monitoring capabilities that may be related to this, but that shouldn't be possible on the Vigor 2766 routers, as they don't have SD-WAN that I'm aware of.

Have you ensured that the right number is entered in the UDP defence field? Too low a threshold can block UDP-based services like VoiP.

Here's how to work out your UDP threshold:

Threshold = connection speed (Mbps) * 1,000,000 / 8 / MTU
2 000 = 24 (Mbps) * 1,000,000 / 8 / 1500

The above is taken from Draytek PL website:

https://draytek.pl/przyklady/firewall-ochrona-udp-flood/#przyklad