[Nilrem]

What I mean, is if the person who may well be paying for a lower service, but using a dodgy box calls for an engineer due to a problem with say the line.

Not send the engineer out specially, but a note on the account to keep an eye out for signs that there has been any fiddling with the service if he's called out for a routine problem, and a note of exactly what is meant to be in use at the address (how many STB's etc).
For example Knockoff Nigel (not just DVD's any more:p) has VM's 3 most basic services, but he's not very with it and has a dodgy box in addition to his normal one - he has a line problem that requires an engineer to be called out, but not being too bright he leaves it connected in one room with his modem (or leaves the splitter in place).
The engineer calls round, and checks the levels on the lines* and having seen the "services" note logs the discrepency, and informs control when he gets back in case either the notes he got were wrong, or there is a theft of service.

Of course if VM already do give engineers a note of the exact services it's redundant**, and I would hope that the engineers would already keep an eye open for dodgy connections to the grey box etc (in the same way meter readers are, I believe meant to note if the gas/electric meter shows any signs of being tampered with).

*Something the better engineers seem to do for all the lines anyway whenever they deal with a signal problem on one (I guess in case that change affects the other lines).

**I know in the past I've had the odd engineer out who wasn't aware that we had 3 STB's on our account when we've had to call them out, and assumed the third line was just for the cable modem.