Received an email today with the info, can't find it on the website though.
WORLD RECYCLE WEEK
Join in a global fashion movement for the planet and help us collect 100 tonnes of unwanted textiles in UK!
Bring your unwanted clothes and home textiles between 18th and 24th April 2016 to any H&M store and receive two £5 vouchers off your next purchases when spending £25 or over!
I took two standard supermarket bags of clothes last year and it worked fine. Vouchers had a long bb date, like a year or so