In a word yes there are huge holes in the network. Anecdotally a friend of mine who is as honest as the day is long managed to forget to MOT his car. He's been driving it for over 18 months between Stoke and Nottingham almost every weekend and wasn't picked up. We were chatting about it and anyone doing local runs and sticking to the side roads could go for years without detection.
There are several issues with the system, the obvious one is that it makes detecting those who choose break the law less effective as you're less liable to get caught. Thus more are tempted to dodge payment altogether. On top of that is how the tax is applied anyway. Why should someone who tootles about and does 3000 miles a year pay the same as someone who does 40,000. You could stick the tax on fuel if it wasn't for the fact that they've already rung every penny out of it to begin with.
So another win for the incompetent folk of Westminster wasting spending our money wisely. £107,000,000 a year down the pan. Was it not supposed to be saving 8-10 million a year?