Hi Ann and welcome to GoY, prune them in March they will grow a completely new cane to fruit on that year.

I think it was Monty Don who reckoned you can leave one or two brown (not grey) canes on when you prune next spring and these will carry fruit in the summer as well

Not what I have experienced Andrew...

I haven't tried it MG. You'd better write to him

I am afraid that by default, I have done this (we unknowingly planted summer and autumn-fruiting canes together which were bought in a mixed bundle from Woolworth's just before it closed down), and yes, leaving canes on will allow it to produce fruit the following year, but frankly, it wasn't worth it: the fruit was sparse and undersized.

I prune mine after fruiting (ie now) to avoid wind damage - we get a lot of gales here.