OP is in the UK, where it is common for phyllostachys to be falsely described as a clumper, even by respected television/radio gardeners. Several of the more popular phyllostachys species are not very rampant in the UK, but they are still runners and will run eventually. I would name Jungle Giants, Hardy Bamboos and The Palm Centre, as respected bamboo sellers who use the word "clumping" in relation to Phyllostachys, (even though the second of these is run by a man who wrote a book making quite clear the true situation). In terms of accurate advice (when selling) I would name Pan-Global Plants of Frampton upon Severn, and Whitelea Nurseries, alias UK-bamboos.co.uk