Stautory Pay or Enhanced?

I'm working out my redundancy notice at the moment... no quibbles about selection, basically the contract is ended and the whole team is out, but my proposed payout appears short because it is calculated from the statutory minimum of 1.5 weeks pay per year of service (over 40) capped at £489 per week and 20 years.

There are no specific or individual terms in my contract but the Redundancy Policy says

• 1 week per year of service (under 40)
• 1.5 weeks per year of service (over 40)
• Capped at 20 years.

So I think the £489 cap shouldn't be applied - as the policy enhanced terms override the statutory minimum. It is definitely an enhanced term because under 21s only get 0.5 weeks per year in the statutory calculation and this is upped to 1 week by the policy (I can point to a precedent).

I've challenged this and the HR response is basically "we've always applied this calculation so it's an implied term in the contract". I disagree - my basic research suggests an implied term has to be common in the industry and can't just be one side's custom and practice.

Has anyone else come across anything like this? I'm prepared to go to ACAS next before tribunal but not if the chances are slim.