Seen them caught on spinners from Cresswell beach, one evening last year when it was realy quite dark. I think it's the usual case of being "in the right spot - at the right time"
Mackerel will show up just about anywhere where they chase their 'food', but if you want to target them specifically from Hendon prom, then I would choose an evening or early morning high tide. No doubt you will catch them in the right spot sooner or later........just a case of keep trying I'm afraid.
Best of luck anyway, let us know how you get on. First ones should show within about 8 or 9 weeks if we get the right weather conditions (Flat seas)