Your redundancy should be based on a 5 day week as you are under 1 year on short time. If the 3 day week continues for over 1 year and you have not requested (in writing) a return to a 5 day week during that time then your redundancy could be based on a 3 day week.
The payment is 2 weeks pay (capped at 600) for every year worked plus 1 week plus any holidays due.
For someone with 6 years work they should receive 13 weeks pay +hols