Gordon MacCulloch was wrong about Matt making it all - not unusual. I think the last verse above is indeed what Gordon added.
Morris told me how he wanted to get Matt's help with verses but Matt was reluctant. Morris sneakily pretended he was stuck for a word in the chorus he had worked out.

'Dum dum dum, dum dum dum, send them back where they belang - Matt, I can't get what to say there. Can you think of something?
Matt said rather doubtfully 'Boomerang?'
'That's it' said Morris.
Matt, cheered, became enthusiastic.
We amended the lyric a bit when we recorded it as the SCND Buskers.
For example:

Chorus:
Boomerang, boomerang, send them back whaur they belang
Send back Polaris, send back Cruise
Give them their orders and tell them the news
For we dinnae like gifts that go bang
Just try one and see if I'm wrang
The banners are wavin, wha's next for the shavin
Lower the boom, boomerang

They say that the rocket ships doon at Dunoon
Belang tae a big millionaire
Ah hope he's takin them hame again soon
Afore we go up in the air
Big Ronnie Reagan's too fast on the draw
And too slow at thinkin it out
If he's too deaf to hear what we're sayin
It's time that we started tae shout

BOOMERANG!