wheel - 3:58 pm on Jan 12, 2011 (gmt 0)

YOu said:

Project was initially agreed on a fixed cost.

But what should really be clear is:

Project was agreed on a fixed cost.

You either had an agreement or you didn't. As long as you didn't change the conditions, the agreement should stand. Sounds like they're just gouging you.

I'd use that as a barometer as to what to do.