UPDATE: I stopped using the 'hold' command and indeed things have calmed down. A case of 'less is more', I'd say.
SECOND UPDATE: With Uly still being reluctant sometimes to give up his retrieves (prey), I've decided to get tough. It's been going on for long time now and whilst steadily improving I feel he is old enough, mature enough to be delivering properly. Today, this week, I am determined to crack this and I'm going to use it by administering the 'Bed!' punishment.
THIRD UPDATE: Had the chance to administer 'Bed!' punishment this morning when Uly decided to start play chewing a dummy he was retrieving to me. In fact, I speeded the process up by getting him to retrieve his most exciting toy-thing, a sort of blue tennis-ball-on-a-rope (I should say that he does not have any of his own 'toys' as such, they all belong to his master (me) and he gets to retrieve them at certain times, I know this may seem a bit unfair to some dog-lovers but it seems to be the done thing when training this type of dog).
Anyway, he was duly told off and slunk off to his bed per the command.
Half hour later I again got him to retrieve the blue tennis-ball-on-a-rope. Result? Well, he was confused and dropped the thing on the way back, he was clearly worried he was in trouble for picking it up or that he was going to get told off again.
So, without further delay I did a couple of retrieves with a plain dummy – with plenty of encouragement and a light tone to my voice he brought them back perfectly and I gave him lots of praise. Next, I tried the blue tennis-ball-on-a-rope again, gave him lots of encouragement and he brought it straight to my feet and dropped it. Another retrieve and this time his confidence is coming back, with encouragement he held it and delivered it pretty much to my hand. I think we may have got this cracked!