Why, if you didn't want an omega due to cost and faults, did you go and buy a similar and reputably less reliable car, with similar parts to the one just sold?
Remember, Oof is in the fortunate poition only concentrate on one model of one manufacturers range of cars. Not all cars have a heater by pass valve. So a factors is even less likely to know (Andyc aside) than a Vx dealer parts man who. They often ask the same question when hearing the letters hbv, ime.
What are the odds of getting a genuine part from a factors though, would you say?
Because the 3.0 omega was falling to bits remember

Rot in the front chassis rails and rear arches plus the running costs and lifters that needed replaced and rear suspension knackered . I had no idea that valve stem seals were an issue on some vectra B's , i had 2 before and never had any issues with that before . If money wasnt so tight i would have another omega but until then ill make do with what i have for now and fix what needs doing bit by bit .
I would have thought any car parts shop would know what a heater valve was and could get one , seems not apart from online stores where you can get anything you need .