If it's within 50m like you say, then it most likely is your fibre cabinet. They don't have to be sat right next to each other. They can be on the opposite side of the road/round the corner, or indeed the next street away. It all depends on local area and what's available in terms of space, underground pipe works, power and sometimes permission, all of which have been mentioned above.
I wondered if this could be the case and you've probably answered my question.
Just to clarify - My cab box looks as though it sits on it's own, is numbered in big letters (twice!) as per the BT checker is of the same number and is of the old type - this is definitely my cab (also confirmed by BTOR engineers dealing with previous faults).
So I'm guessing the FTTC cab that also has an old style cab next to it too and is approx 40metres up the road, basically serves my cab aswell.
It makes sense to me that one FTTC cab may serve more than one PCP if they are all within a certain distance whilst still within expected FTTC demand numbers. I have just been used to seeing new FTTC cabinets usually reside next to an established PCP's and assumed (wrongly it would seem) that was the way it was done.