Apologies if I get any of the terminology wrong here.

I have a 2820 at one of our sites which we have just taken over (this is the router that was being used by the previous owners). We have a VPN going from the router to our WAN (which is supplied and looked after by a third party) and for the most part it works perfectly. Most of our sites on the WAN are in the 10.1.*.* IP ranges and I can pass traffic across the VPN OK.

I have one server on a 192.168.1.* IP range (I have several but it just this one I need to be able to connect to devices on the site side of the VPN) that is unable to pass traffic through the VPN and for what it is worth I am unable to to ping anything on the 192.168.1.* network from behind my VPN

The support company that supplied and support the WAN have spent some time with me trying to get the router to allow traffic over the VPN to the 192.168.1.* but to no avail. They are happy with all the setting they can see on the Lan to Lan VPN setup and the routing table but they are not Draytek agents so cant say with any certainty that all the setting are OK.

Can anybody suggest where the issue could be?

L2TP doesn't support routing, try to get a Cisco IPsec tunnel if you haven't already got one.
If it is an IPsec tunnel, you can simple add a static route to the class C network.