I dont know about that, my boyfriend got a letter and calls and a notice posted on his windscreen a few weeks before the 6 months was up, so somehow they knew. Cos if its taxed in Poland you dont have a tax disc on the window, so its obvious. Then if you try and tax it here, they register it.
Your boyfriend was shopped by someone (neighbours probably) as there is no real system in place to monitor foreign registered cars and it is purely down to someone telling the local DVLA office that a car has been around for ages and for them to look into it.
I'm not suggesting that your boyfriend was trying to avoid registering his car, but at the same time I don't blame whoever made the call for doing so.
probably your neighbours rang up the DVLA or council to complain. I would be interested to know what happens if you ignore these letters etc.. because I don't see how they can prove how long the car has been in the UK.
Personally I don't see it as a "complaint". Either buy a UK registered car if you intend living here for a long time or register a Polish car with a UK registration number and pay what the rest of us have to pay.
The DVLA would go by the date that they were informed about the car being here. Six months after that they would be able to take action, unless it can be proved that the car had been out of the country for a long period, which might re-set the six months back to zero.