First pool shop I went to (North Shore) said I needed a whole new backup valve unit - $160
Second shop (also North Shore) said any collar I purchased would be aftermarket not be an exact fit, I would need to buy a new shell backup valve shell, which I happen to have in stock - $50
First google search brought me to poolshop.com.au - an original equipment Polaris collar was available immediately, $18 - I took a drive down to Warriewood, NSW to Poolshop's storefront, made the purchase and have fitted the collar - now I have a perfectly functioning Polaris 3900 again.
Very unimpressed with the attempted gouging by the other pool shops - with a little effort they could have had an original part supplied from Polaris and a happy customer. I will be recommending Poolshop.com.au to anyone who asks - 10 out of 10 !
excellent and fast service, will certainly use them in future