Redundancy is compensation for lost future earnings so if you are working 2 days a week now your redundancy payment will be based on your weekly hours and pay at the date of redundancy. Your past earnings are not relevant..
However the 20 years service should count in any redundancy payment scheme which reflects length of service.
So for example if your scheme gives 2 weeks pay for every year's service and you now earn �200 a week (based on 2 days a week) your redundancy pay after 20 years should be 20 x 2 x �200=�8000.
Statutory redundancy works in a similar way. Does this help?