Recommendation is to 'divide large clumps in early spring'. Which usually means digging up the clump, splitting it and replanting the sections, keeping them well watered throughout the growing season for the first year. Alternatively, look for some outgrowth you can separate from the parent plant, preferably with some rhizome root attached, pot up and plant out next year.

It's now too big for me to dig up. (no muscles)! I'll have a look and see if there is any outgrowth. Would I simply use a sharp knife for that?

Yes, hopefully - a good breadknife works quite well!

Thanks Bamboo, that's tomorrow's task sorted.

Be prepared for it to take a while to get going. I think mine went into shock when I cut into it.

you mite need an axe to seperate it and it does take a while to get going . on the other hand its quit natural for some bamboos to seamingly get very congested . its the plants way of stopping anything else growing there and competing . mite be worth just putting a bamboo membrane round it to keep it basicly wear it is . it will be fine .bamboo is some things to some men and all things to some men . its the most durable plant on the planet and has a documented 2000 different uses and some . it was even used as an element for the very first light bulb .