So IT Checked and double checked the switches and could not find anything... then came a stroke of random genius, sort of.
Basically the Virtual DC is on a Hyper V cluster made up of 5 servers, the DC was on node 004... I moved it to 001 and suddenly the server joined the domain. I guess this kind of explains why it was working and then has stopped however this obviously shouldn't be an issue so that will need to be looked into seperatly.
It's also the reason the other machines were connecting fine first time, through freak coincedence they were all on the same node. When I moved the DC to another, one of my test clients could no longer ping.
Thanks for helping, not sure anyone would have really figured out our virtual clusters failings as it is probably the last place you would think to look during all of this.