Glueing temperature degraded second hand broken plastic together won't work without some serious preparation. By which time you will be better off replacing with new. And even if you do bodge it together it won't hold for long. It broke in the first place because it's a weak spot.
Also, if that obviously broken piece isn't attached to the spigot, then it's in the housing, and it broke in there for a reason.
No offence, but you're old enough and bright enough to know better 
All of that.
Plus, sealant isn't intended to join parts, you need an adhesive. Two part epoxy is the thing, but you'll probably want to pick one based on its specifications rather than pretty packaging in Poundland.
Gluing broken 12 year-old plastic coolant parts together is a will it last long enough to get a new part measure.
Thanks to all for advice. My two local main dealers who stock and might deliver spares, Drive Vauxhall Bristol and Platinum Vauxhall Trowbridge, are closed until further notice. Is there any online way of buying that pipe?
I don't need the car at the moment, as my Omega MOT has been extended for 6 months. Coronavirus patients over 60 are not put on ventilators, and over 75s are not even admitted to hospital, so my obvious course is to SORN the car and forget it.
I managed to remove the end of that pipe stuck in the thermostat housing, found no 'O' ring on it, puzzling. I now have nothing to lose attempting an epoxy repair, it might last longer than I do.
You recommend thoroughly cleaning both parts with carb cleaner or brake cleaner, I find I have both. Should I roughen both surfaces to give better keying of the adhesive? I find I have Araldite Standard (ultra strong), Araldite Rapid (5 minutes), and Ever Build Stick 2 Rapid Epoxy Syringe Adhesive. Obvious choice is the Standard. Please advise.
As I type this SWMBO is crawling round the floor playing with grandson, his father is upstairs running his online business and his mother is in hospital having an infusion for her colitis. Surreal, ain't it?
