It is based on your normal hours at the time you were made redundant. If your normal hours are now considered to be full time, then your redundancy will be based on your full-time wages.
If however your extra hours are only considered overtime, then your redundancy will be based on your old normal wages with an additional amount calculated for overtime. To calculate overtime they use the 26 week period ending 13 weeks prior to the termination of employment.
See the following guide for more details: [broken link removed]
There are specific different rules about piece workers (ie, workers not paid on a time basis) and shift workers which are set out in the above guide too.