Q: How do I fit these adapters ? Do I remove the existing ones or they go on top
A: You remove the existing TRV head. Screw on the Adaptor, then screw on the electronic head.
Q: Is this the correct adaptor to fit a drayton wiser head to a Pegler Terrier TRV 662206 discontinued
A: Yes it is.
Q: Is this the correct adaptor to fit a Drayton Wiser head on a Pegler Terrier 3 TRV?