I dont believe that there are any regulations regarding the usage of HRM in road races. The only reason the elites dont is because they are so in touch with there body that they are able to judge maximum sustainable effort very precisely. Some believe even more so than HRM's, as this can vary dramaticly for the same percieved effort.
Also, on the basis of electronic as cheating, almost every professional marathon runner in the past ten years has 'cheated'. Think about the scene as the elite line up at the start, hands on digital stopwatches! An electronic pacing aid if ever I saw one.
So to conclude, you certainly didn't cheat! You should be duly proud of your achievement, hrm or no, expecially on such a congested course! I took longer than 51min on the Jubilee 10k, & I normally run 10k's quite a bit quicker than that so such a time is no mean feat!
KK