Didn't the Cavalier Coupe pre date that generation of the Manta? If my dodgy memory serves me correctly that generation of the Manta was designed in Luton, and used the Cav Coupe and Sportshatch as its base, and not the other way around.
Would the Cavalier coupe /droop snoot have been equivelent to a Cavalier Mk2 ?, the early Manta was (I think) a purely Opel thing around before the Cavaliers, a much earlier design, quite european looking, later Mantas followed with later Cavalier models of course. I'm just musing, I have no idea really

I had the 1978 or 9 Opel Ascona Berlina 2.0 SR, (basically a Cavalier Mk1 design) I think the Manta was was a stylised and slightly higher performance rated car based on the ascona. The 2ltr went well but as mine was relatively old even then, so I didnt hammer it .
Sorry, I'm distressing from the OP.
Astra GTE certainly has a following, as do most hot hatch cars from that era.