If you are visiting Runswick Bay or any other sandy UK beach you need to know about this; the Weever fish, a member of the family Trachinidae:
The sting is painful, it is not fatal and it is very easily treated; PUT THE SITE OF THE STING IN WATER AS HOT AS THE CASUALTY CAN BEAR. The pain will diminish almost immediately, but keep the treatment going for around 15 minutes. Watch out for shock; Runswick Bay's Weever victim was fortunate to be treated by an experienced first aid instructor from Barefoot Kayak (which hires kayaks and paddleboards at Runswick during the summer holidays) who spotted the shivering and wrapped him in a thermal blanket.
Weever stings are rare, don't let fear of them stop you enjoying our lovely beaches. They are most common around spring tides in August; as a precaution don't paddle at the low tide line during spring tides and if you do wear shoes.
We didn't get to see the offending fish, but it was probably a Lesser Weever (Echiichthys vipera).