Did the encounter give you any indication of whether Ziggy was aggressive to the cat, or just overexcited?
Our situation may be slightly different in that my dog Jasper arrived at 7 weeks old when the cat, Monty, was 12, and J was besotted with M. Besotted in an 'I want to chase you, wrestle with you, play bitey-face and [when he was older] hump you' M was not impressed, but also never stood up to J. M is just a week shy of 20 now and pretty well lives in the walk-in airing cupboard and bathroom (behind a stairgate). On the rare occasions he does venture downstairs J is usually good, but because it's such a novelty, he can still try the charm offensive and with J being a large dog and M being extremely frail, this doesn't go down well. I would have worked more on their interactions, but as J got more mature and responsive, M became more frail and I didn't want to put him into a situation he'd find stressful (such as bringing him into a room with J while J was on lead). He's adjusted to his new life and gets plenty of cuddles whenever I go to the loo or sort stuff in the airing cupboard, and chances are he wouldn't be doing anything more than sleeping in warm places without J in the house.
My suggestion is pretty much the same as LoQ's below - keep working on rewarding calm interactions with Ziggy on lead, but take it gradually so (ideally) he's never aroused enough to react. And expect it to take as long as it takes