l need a firmer centre for a caramel slice
You probably need to bring the caramel to a higher temperature. The Bon Appetit recipe calls for the caramel mixture to be brought to 225º F. (~107º C).
You could add less liquid (usually in the form of cream). This can sometimes make the caramel too firm though, so you could add a little bit of butter to keep it solid but soft. I can't really help you with quantities unless I see your recipe but I would start with adding half the liquid and 10% of the caramel weight of butter. You can make the caramel in advance to check for consistency because it can always be softened (slowly and carefully) and altered, then poured into your dessert.
Adding a touch of acid (like lemon juice) can also lend a bit of softness to the caramel.