no sig,
I would say that as per the UK AIP any occassion that falls within 30 minutes from the CTOT should be handled locally. Like nodelay, I have had at least three incidents where ATC locally have negotiated an extension on a slot with the FMU only to find that the relevant ops department have at the same time either entered a delay message or sent a re-route. All three incidents resulted in the aircraft receiving a lengthy delay and having to return to stand after almost being at the holding point.
It seems to me at times there is no communication between crews and ops as ATC are being asked to do one thing, while ops are doing something else.
As someone else has pointed out, the FMU are normally very helpful and bend over backwards.