Definitely sounds like a case of number plate cloning to me, particularly as you have had 2 in a short space of time. Clearly whoever is driving the cloned car doesn't care about picking up tickets.

As above, while you continue to try to prove your innocence change the plates on your car to one's with a welsh flag or something asap and record proof of this and when they were changed, then if they happen to pick up another ticket anytime soon you have pretty good evidence that it's a clone. Also, the fact that you mention the plates alreayd look slightly different to yours (the lack of dealer name) seems a pretty good route to explore when arguing with the safety camera people!