Yes the CS92A is in the closet with the boiler and the hot water cylinder. (Although it's only a small 100 litre cylinder) It is 70cm from the nearest BDR91 and 50cm from the cylinder. Distance to the controller which is permanently wall mounted is 6 metres with line of sight through the closet wall and a glass door. Signal test is 5/5.
I've already moved the CS92A as far away from other objects as possible - moving it further away from the BDR91's last year did help quite a bit but it is still not completely reliable.
Perhaps, but only if it is faulty from new. You only have to look on forums to see how common these symptoms are though. I had a look on the evohomeshops own forum a couple of days ago and found 3 threads in the last few months all for the same problem I'm seeing of intermittent hot water overshoots. So I doubt it is an isolated experience given the relatively few threads on that forum in total.
The level of troubleshooting that I've already performed in the last 9 months is already well beyond what would be expected from a normal end user so I think I'll email Richard and ask him if he will swap it. (I think it's still within the warranty) I'm sceptical that it will help though. I think the problem really is just a result of the less than 100% reliable communications protocol in anything other than perfect and unrealistic RF conditions.