Sorry not to good on the whole explanation thing,
...we had to switch the DHCP role from the dead DC to the other DC. We restored the our redirected desktop and start menus from a backup, to the second DC and corrected the GPOs to redirect to the second DC. Once the broken DC had been nursed back to health, the DHCP role was disabled on the second DC in favour of the original DC.
We then started to have clients refuse to log in, claiming the net logon service was not running. Resetting the netsh winSock corrected this, but each time the DHCP lease expired, the network interface crashes and needs to be reset.
In addition to this, sporadically PCs cant access the internet (DNS issue?) and GPOs and net logon scripts are not being run or correctly adhered to. Staff are getting the wrong redirected desktop for example, despite the GPOs being correct.
Also... when opening the Group policy management console on the formerly dead DC, were now presented with "The specified domain either does not exist or can not be contacted", which we can only assume is not good.