Have a look at products like ''Root Barrier Bamboo 420''. Lots of custom-made products on the Internet.

Sounds like you have one of the runners and not a clumper, Jimmy’s right you need to put in a barrier, if not it will just keep on running, this is what I have done in past, old large size oil drum, cut bottom out, dig out ground and sink it in, then half fill with decent, soil compost and mixed with gravel for good drainage, then plant, after so many years lift out and reduce it a little, do this before it gets completely rootbound as it will be difficult to do then, I will be doing one later this year and will try and get it on here for you to see.

Thank you Jimmy, I shall certainly search this product out, sounds perfect.

Thanks also, Julien, if I get desperate I'll drag my son-in-law into doing this for me.