Hello monoman, I had exactly the same problem. The reason it engages down hill is the handbrake works off a small drum and the direction pulls onto the shoes . I was informed by the dealer that there is a small drum buried inside the disc and small brake shoes operate on this . Initially the dealer replaced the electric handbrake motor , but was not that good ,a bit inconsistant . So took it back and asked the workshop manager to come with me and try it on some hills and indeed he said it was not good enough . Said they would ask vauxhall for advice and leave car with them and they would act on that advice . 2 hours later he returned car and said they didnt know about the drum brake , but they do now and they had never had to touch a antara hand brake . In his words a very well engineered system . We took it to a 1in 5 hill near where i live and it held rock solid .
So i bit long winded getting to it ,but go back and tell the garage it as a drum .
Also question vauxhall information , i did and they told me antara definately got a small drum handbrake . My antara is a 2013 year ,so if yours is not a earlier model it should be the same ,as i dont know if it applies to the earlier ones . But as i say ask vauxhall info site .
Hope this helps ,