Having removed the captive nuts from their cages on the front wings I took the opportunity to cook the offending articles in the oven (my wife was out at the time). Still no joy so I took my angle grinder to the remants of the adjuster screws leaving them flush with the nuts. By gradually increasing diameter of drill bits - up to 4mm in the end, all remaining screw swarf was surgically extracted and lo and behold the adjuster screws operated perfectly.
Of course now that I have done all of this I think I am right in saying I won't be using all of the captive nuts anyway - especially if I decide (and I haven't yet) to go with plastic headlamp bowls. But I was pleasantly surprised that a bit of perseverance and patience paid off (for once!)
