Our lan 2 lan vpn works fine apart from the inability for new users on the remote side to login due to a lack of domain controllers being available. If I login as a previous user (when the computer has logged in on the domain controllers lan) I can get in through cached credentials. I have added teh domain controllers details in the lmhosts file and the computer can ping the remote server on its private IP and can resolve its name.

can you resolve the DC from the remote site by name?
We usually get around this by setting the DNS server for the remote site as the IP at the main site where the DC is hosted.
Failing that, install a DC at remote site