Judging from the history, you are only likely to suffer a check in the event of your being involved in an accident, and even then only if someone was injured or killed. That's when police traffic officers get involved and some of them do know pedelec law. However, if any 350 watt markings on the motor are removed without trace it's unlikely they'd investigate the motor further.
If the bike you've bought is a high speed machine and you are seen bowling along at near 30 mph without pedalling, that would invite a stop by traffic police, and probably result in the bike being confiscated since it would in law be an uninsured motor vehicle.
.