A broken and/or worn brake pad sensor wont trigger the abs light. It only shows up a orange light which is a circle with some dashes on each side of it.
If your bolt is worn off might be worth returning the new abs sensor and get a new hub complete with 1 already fitted. Astra/zafira hubs are about £30-40 retail (thats what i paid several times) so vectra one should be around that price mark from your local spares shop.
Traders place might do it a bit cheaper if your in the trade or good at negotiating.......