I would like to think the ATC I work with do have a good idea of what I'm ale to do, but occasionally I'm shown to be wrong.
Manchester have a habit of slowing us (146) down in the WAL area on approach to IOM to sequence us with far slower J31s and the like. I do wish they'd fan us out and let nature take it's course instead. This last few months, this initiative has seen us arrive at KELLY far too high, above and in front of the preceeding traffic, despite slowing right down.
It's just one instance, but shows that the awareness of what we do is lacking in some controllers regarding the relationship of TAS to IAS and MACH. I am often asked on quieter apps to LGW if X miles is enough, but for a 146 a 2:1 is easily manageable with little discomfort down back.
Unfortunately, all we can do to attempt a CDA approach is refer to the booklet that was sent out as we have no VNAV data available and it's purely seat of the pants. A second or third range check would help here, as would not sending us on a tour of deepest Kent.
Generally, I have no complaints, and as I once learned from an Ex Police Advanced Driving Instructor, the important thing is that one of you takes the decision, more than who it actually is. ( the subject was overtaking a parked car in oncoming traffic BTW)
If the crew are uncertain in their reply is the distance is enough, then make the decision and give them another two or three miles anyway. If they want to orbit in hilly terrain, rmember you mum might be down the back and by all means tell them to do a missed approach. Better if you can offer a vectored circuit to reposition though, time and workload allowing.
It's uncertainty, misheard communication and indecision that frequently forms the chain of events in accident investigations.
Positive control where it is obvious has saved many a life. Dithering has caused many a death. If I as a pilot am unsure, and you the controller are not, them be decisive and make the safe decision for me. We can always chat on the phone later if need be.