Reporting back in in case anyone's interested...
I did get the gun apart. Fortunately there was no glue, only screws. I then had to work out how to get the slide off (slowly and taking pictures as I went!). Eventually everything separated. There was a tense moment as a spring popped out and I had no idea where from.

I carefully put all the pieces in a box and left it alone for a while!
Over the next few days I inspected the pieces and, one by one, figured out where they went and what their function was. Never having done this before I had no idea how airsoft works, but now I think I do.
The mechanism for the electric blowback is complex (lots of gears and things) but also stunningly simple in concept!
Last night, to see if I had it right, I crammed all the pieces back in and clipped the gun halves together. Put in the batteries and to my relief, it still worked.
Oh, one tiny thing - in it's half-assembled state I seem to have accidentally converted the thing to full auto! Oh dear.

Don't worry, it will go back to normal when I restore it to its original state!
Mike