Thanks for the reply. I checked all the buttons at the time, each time it happened, but couldn't find an obvious problem. That housing had to be replaced by Ikelite when it developed problems with the flash circuits, it just died one day, so it may have just been a very dud housing.
I have had several Ikelite housing and have always been happy with them, but had one that wouldn't funcition in depths beyond 35m. I could press the shutter release, but couldn't change any camera settings. This happened each time I went to depth with this housing, and as soon as I got above 35m everything worked fine. I have used my other Ikelite housings down to 50m without an issue, every button and dial worked perfectly. I contacted Ikelite and they told me that they had never heard of this before, but it now sounds like I am not the only one to experience this issue.