There's a huge difference between going to litigation and just asking a lawyer what your legal position is (which needn't take too long or be too expensive) so you know the rough shape of what your 'worst case' looks like. Most businesses are reasonable and willing to do what's 'fair' even if that goes beyond what they might be legally required to do because employee goodwill is worth a lot to them and the cost to them of litigation is also high. But they will also have an idea of what their legal position is and that will inform their idea of what 'fair' is. A good lawyer will tell you what they'd expect you to be able to negotiate commercially and tell you to try that first.

Your employer doesn't even need to know you consulted a lawyer.