They work better from layering rather than from cuttings. If you have a spare rod lay it on the soil, peg it down and it usually roots either at the tip or from every leaf joint.

I agree with OB and would go for 'Tip Layering' if you don't want to disturb the doner plant. This would be the most reliable method.

Thanks guys, will do it this afternoon if it’s the appropriate time of year?

It's ideal time of year, roots will form over winter & all ready to go come spring

My thornless blackberry arrived from somewhere in my soft fruit patch and there is plenty of foliage growth . It is very sturdy but the fruit is quite small and difficult to pick .
Is that because of this particular year , or the nature of this odd origin plant ?

Triad, sounds like yours is a stray seedling, if it's the first year it could be because of this particular year being so dry, my blackberries failed to get to a decent size this year, as have many of the hedgerow ones

However, if it is an older plant I'd be inclined to put it down to genetics and get a named variety with known characteristics

Thanks all, have buried a “ spare rod” and will keep a careful eye on it.