Yeah - sorry, I eventually got this sorted and should have posted back.
I was pretty much correct with the diagnosis - the problem was caused by BT Openreach. When cabling me up for Infinity, the engineer had plumbed my fibre connection in the cabinet into the wrong port. The port belonged to a neighbour**, so BT's Usage Monitor was reading my neighbour's usage instead of mine.
It was a mare to sort out. BT initially closed my call after 7 days without doing anything about it; no communication with me whatsoever. On opening another call, it then took me 5 x additional calls to 1st & 2nd line support to convince them there was a crossed line issue, and to book an Engineer visit, who only fixed the problem after I explained to him all the details (he didn't have a clue about the problem details when he arrived!).
Subsequently, I had to call BT Billing to ask for refunds of £31.80 (from December's surcharge) and a further £21.20 from a surcharge incurred in January. This was promised as a credit on February's bill, which never materialised. I then wrote a snotty email to BT Customer Services threatening all sorts of action if this wasn't corrected within the next 14 days and this finally led to a satisfactory resolution.
** I found this out when - after going round in circles with BT 2nd line support - I invited the support guy to 'remote in' to my router and read out what devices he thought were connected. After reeling off 4 x devices, none of which I owned, the penny dropped as to who owned said devices and therefore what the problem was.
So, about 6 hrs spent on the phone to the Call Centre and a heap of frustration and stress....but that's for another thread! In relaton to this thread, I just thought it might help the OP with his problem as it sounds like it could be something similar to mine.