Hi all, decided to try a different solution to Draytek, as only really wanted it for reliable LTE failover (which it does magnificently) - but not much use for being able to browse all websites. Just don't have the time to pursue this any further, particularly as it seems to be an issue far beyond the our ISP.

So I've come back to give the solution to our original problem. Now back to using Draytek, as the reason for the original issue it turned out was our domain host had blocked the WAN IP address due to multiple failed POP3 email logins (don't know where they were coming from) thus preventing access to only our websites. Before discovering this, tried a Billion router which had the same issue, finally spoke to domain host who explained and then fixed the issue in seconds by whitelisting the offending IP address. Preferred the Draytek, so have reverted to using that.

FYI if using a Virgin Media Hitron Chita router, just be aware that in bypass (modem) mode, of the 4 ethernet ports on the back, only any 2 ports can be allocated for WAN connections, and VM will issue a different dynamic address for each port (that's the limit, a 3rd/4th WAN connection is dead). This is not widely known, found a reference to it on a Canadian website as Rogers (Canadian ISP) use same s**t router.

Once again, many thanks to those here that that did what they could to help - much appreciated