I don't think we have ever had this problem, Alan. Since it is the first time it ihas happened to your plants, would I be too optimistic to hope that it has been a one-off incident?
I would think that caterpillars of some sort are the most likely culprits.... and it might well be that there was a population explosion of some kind last year that allowed this damage to occur. Of course, the follow on to that might be that this year, after a not very cold winter, there may be more of these critters about to do the same again.
Wearing my cheeriest hat, I hope that the one-off scenario was the case.... but it may be that you have to be extra vigilant during the summer to watch out for the beginnings of further damage.
If you identify an attacker you will then have to decide whether or not to fight back with chemicals ..... ah, me, who said gardening was all about relaxation?