I've clarified now and basically you can return any time, but if it's within 6 months you have to repay the redundancy, which is fair enough tbh.
HR lass suggested that although I can sign the settlement up to the end of the month, sooner means the PILON is more likely to be paid end of this month rather than having to wait for March payroll, but I'm thinking if there's a possibility (and there is) of an internal role coming through before mid next month, maybe that's better, as they might have time to stop that extra sum being paid out only to be repaid!
I've just heard from someone senior there might be a few of the roles I'm after available soon as there's a few people moving around,, and he says to keep chasing up relevant hr person, who I'm sending my CV to today (we're such a huge business that each part has its own 'people team')