after completing the broken circle quest and killing that dude at the end, the game glitched and took me back to the start where you have just come in the door with the knight commander, however the door is locked and there is nothing i can do apart from trade with quartermaster or leave the building, what do?

What system are you playing it on?

vista, never had the problem when ive played it before though

This post contains a very small spoiler. 
 
This is apparently a pretty serious and known bug.  There is no known fix but I have a suggestion.
 
If you haven't done the Redcliffe part of the game yet, go do that quest set.  At the end of that quest line you have to make a decision.  One of these decisions states very clearly that you'll need to go to the mage tower and try and get help from them.  This may force the dialog on Greg to reset as he should tell you the mages can't help because the tower is locked down indefinitely. 
 
It's worth a try... otherwise though you may need to revert to an earlier save... you did have multiple save files right?