KFC are running a promotion for the next few weeks, every Friday, Saturday and Sunday every bucket with 10 or more pieces will come with a free large popcorn chicken (worth £3).
The popcorn will appear automatically on the till so there should be very little problems with them being missed out of the order.
This offer works well with certain buckets... e.g. 6pc bargain bucket - £9.99, 10pc bargain bucket - £12.99. The price difference is £3 for 4 extra pieces of bone chicken and a large popcorn.
The 20% of with the receipt might work, will have to wait until Friday to test it.