This is a common problem with Boardman FS bikes as they do not lock-tight the bolts, and the rear side near the derailleur seems to be the weak side. The bolt is in two pieces, one side I think is a 6mm countersunk standard screw, but the other is a specially designed part.
I have lost one previously and also had it come lose twice (even with blue locktight applied) so check it before every ride.
The good news is that even though the shops do not have the specialists, their helpline does and believe it or not the knowledge and help I have had from them previously has been brilliant (5 stars!). Call their customer service number. They sell a bolt pack which includes all the pivot bolts for £25. They will send it to your local store where you have to go and pay/pick it up.
I suggest putting the spare rear bolt in your backpack.