I think you need to look in a different genus. This looks like a young columnar cactus, possibly a Myrtillocactus (that's just a guess). I would imgaine it should be quite vigorous if given good water and fertiliser in summer, plus sunlight obviously.

Thanks for that s&b.
But that's the thing, it has been fed and watered in a cactus-esque manner and been on a south facing baywindow getting plenty of sunshine,(well as much as our climate can afford), plus I had it out in a miniture greenhouse all summer this year, so it is receiving favourable conditions.

It might be good to get a confirmation on the ID before we jump to too many conclusions ... but ... I have found that the vigorous columnars like a good root run, even to the point of serious overpotting. Then they go mad and fill up the pot by the end of the summer.

I think I've just found the answer on google images ( typed Myrtillocactus and hit enter) this is what it came up with amoung the images of Myrtill's
Stenocereus pruinosus.

Here is a link that might be useful: {{gwi:490501}}

Newbhoy beat me to it.
It does look very much like a stenocereus pruinosus.
Mine likes plenty of sun and plenty of water in the growing season.
If you are treating it right and it isn't growing well, it would not hurt to check for root insects (mealies and such).

Waz

Waz

Thanks Waz
I'll take your advice this spring and clean roots, change soil and repot, like you say wouldn't do any harm!
Cheers.
Scott