Wouldn't be frosts earlier this year causing this - assuming you've checked the plants over thoroughly for signs of infestation or infection, the most likely explanation is drought - the problem would have been probably around 6/8 weeks ago. Can't tell how likely this is as an answer because you haven't said where you are in the UK

Honey Fungus could do it too, fairly lethal to conifers if it gets them. Difficult to diagnose, though, unless you strip the bark off !!!