Fitting PSI Powerbox (for dummies)

I have a PSI Powerbox from Paddock and went to fit it to my 1999 DHSE today. Pretty sure I located the correct connections but when I come to turn the collar of the large plug anti-clockwise I cannot shift it. There looks to be a 'tag' on top of the collar and I wondered if that might need moving to one side or pushing down??? Can anyone give me a clue? Please!

It's a bit like a bayonet cap lamp, push and turn, Hard to get a good grip on it. Look at the connector on the PSI box and you should see how it works.

Thanks Keith, I will give that a try after lunch.
Bill

Looking forward to your feedback on this unit.