using buy 4 get 1 free rule, and ££ vouchers together ?

[Ray UK]

Hi,

 

We have a rule set up, so £4.99 per item, when you buy 5 they are £4 each, so £20

This works on a per category, so any 5 items from that category, -99p off each item.

 

Now if we make a voucher code for £5, how can we tell it not to take money off that existing 5 for £20 rule ?

 

So the customer orders 5 items for £20, and another item for £54.

I want the voucher to come off the other item(s) but not ones already discounted by the rule.

 

Is this possible ?

 

Thanks