What the Project Manager should have done about five months ago was make their own assessment- which is what clause 64.1 tells them to do. It is actually an obligation rather than optional for the PM to do that. I am not sure what conversations have gone on within that period about the lack of quote but here the Parties should have both done more to have avoided this dragging on so long.
You have two options here - you can either ask them for another quote (and I dread to think how long that one will take to come back judging on the first one) or you assess yourself. Either way it should be assessed using the rules in clause 63.1, which it is based upon a forecast as to what the event should have cost at the point the instruction was given (or for other events not stemming from an instruction from the point at which the CE was notified).
If there were elements of their quote with hindsight you can see they did not use, then you need to consider if it was reasonable that they would likely have needed them at the point they quoted. Equally, if they actually had four resources and it would have been a reasonable forecast that only two resources were needed for that operation then you can base the quote on two resources.
Just to be clear - as explained above Defined Cost in itself does not mean actual cost - as 63.1 talks about elements being based on actual Defined Cost and elements being based upon forecast Defined Cost.