No I am afraid you cannot assume that the little blighters have gone. You would think B&Q would have treated the timber before selling it but you cannot guarantee that they have or that they did it properly. From the photo I can see what looks like the remains of a chrysalis which would suggest they have gone, however the little darlings can remain in the timber for up to four years and the same timber can be re-infected as the females will lay their eggs in old exit holes. I come across this problem on an almost daily basis, in fact I will be treating an 18th century chair today that has both old exit holes and relatively fresh ones. There are several ways to kill the grubs, you can cook them by baking the timber or if it is small enough put it in a microwave however the conventional way is to use a chemical such as Boron or Permethrin. The instructions on the can will tell you to brush or spray it on, I always inject it into the hole with a hypodermic just to be sure I get them all. At the moment I am using lumberjack boron based woodworm killer, It has a slightly soapy smell to so is not too unpleasant to use, some of the formulas, including the leading brand stink to high heaven and if the timber is to remain indoors you can still smell it months later.
Thanks mrpercysnodgrass for your informative and helpful reply, I had hoped the opposite would be true. I have looked at the chemical products and TBH its cheaper to buy some new CLS - not from b and q!. Bit annoying as I had already half built a shelf unit.
With the discolouration in and around the holes I would say they are not from the common furniture beetle. they look more like pin hole borer that lives in the living tree and then dies off when the tree is processed. so may be no need to worry after all?
tho I've not seen it in soft wood before.