Instructions:Saw this in small print on the most recent newsletter I received from them. No other clear T&Cs, just says use code 6019 to receive 10% off and free standard delivery.
Posting for visibility for those who aren't subscribed to the newsletter.
Edit: Just seen that is is advertised on the website homepage as well but still keeping it here for those who, like me, like to browse HUKD for voucher codes before deciding where to buy stuff (e.g. I only ever buy from H&M if there is free delivery available)