Tax Refund using form P50

After being made redundant I have decided to retire early,the payment was just over £100k taxable income. With my redundancy money I maxed out this years Sipp and was given a paye code of 3075l. I completed form p50 to get a tax refund as I wouldn’t be working again (HMRC estimate is I’m owed just over £20k refund). I’ve now been told I cannot get a refund until next years and must complete a self assessment because earning this year were just over £100k. Can anyone confirm this is right as I can’t see anywhere on line there is a cap to the p50 form. And if I do have to wait till next year does anyone know when I would likely to receive the money. Any help would be appreciated.