cooling system header tank

[ChrisG]

This is for my 1985 80 Sport. OK other than a good cleanout and flush I have done all the bits on this, BUT the low water light now flashes continuously.
It should flash when the ignition comes on, then go out unless the coolant level is low when it flashes ( I understand it also flashes if the temperature is very high but as I have a working guage I can see that it is not that.)
Fan cuts in and out as it should.

I suspect the sender in the bottle may have broken ( but was OK before I started draing down , rinsing, new stat etc)

The plug underneath the bottle appears to be designed to unscrew but doesn't.
Any ideas how it comes out?
Is the sender replaceable even?
Otherwise anybody have a working spare
857 121 407
857 121 403
443 121 403
are the part numbers on the bottle.
I suspect this may be used on a range of Audis ( and what's with the 443 number ? )
Thanks !

I've also asked on Audifans.

Guys get out your power steering details as I'm changing the fluid next !!