Hi, I have the Sony RDR-HXD710 and I have exactly the same problem.
When you look at the EPG, etc, you will find that the unit does seem to "know" the right time, because it displays the programme detail that is currently showing.
For example, now it is 10.13 and pressing "OK" to show the current programme will show "10.00-11.00 - Dr Who" or whatever it is.
BUT - the system clock time, as displayed in the top right of the screen on the EPG, or on the unit when switched off, is still at the old time. This is a problem because if you try to set a manual timer recording, it automatically shows you the system clock time to begin with. And heaven knows what it will record!
Like you, when I go to the manual method for setting the clock, it is "greyed out". I have done the auto-set-up business all over again, and even done a complete system reset, but nothing works.
I thought my unit being quite new may mean Sony had an error with the machine - ie this was the first "BST/GMT Time Change" since the unit has been available for sale and it's only now that we find out the bug.
But, surely the GXD and similar models have been around for a year or more, so has anyone had this sort of problem before?
With the GXD in question here, what happened in March when you had to go from GMT to BST?
Any clues anyone??