"The North Wales accent is quite different to more Southern areas and it is a bit jarring, to me, that the accents aren't right for the area. "
The accents are a bit all over the place in this programme. I presume the producers just decided to get the best people for the job. Similar criticisms were levelled at Hinterland that the dialect spoken was more Carmarthenshire than Ceredigion.
The actor for Dylan is from north east Wales, but uses a north west Wales accent in this, very convincingly. The woman who plays his mother is a well-known actress from the south west and this is the first time I've seen her play with a north Walian accent. I thought it was quite good, but of course because I know her real voice, I did find some mistakes. In the Welsh version the student Megan and her friend are south Walians at uni in Bangor, which is pretty common, but I think that in the bilingual version they're not Welsh speakers.
Cadi's family is a mix, something a lot of people have criticised, though it does happen sometimes. The father and the younger daughter have north Walian accents while the two older daughters sound south Walian. I imagine that the family possibly lived in the south when the older daughters were small and then moved to the north, where the father is from. It's quite strange that in the bilingual version we're asked to believe that they're all first language Welsh speakers living in a mainly Welsh language area, but speak English to each other.
The other police officers are all north Walians, but a few of them sound second language. The little girl's accent is also a bit strange, as if she's from another area or a second language speaker, which she wouldn't be in real life growing up with the Welsh speaking father and grandmother and having no contact with the outside world at all. I presume we're meant to believe that she learnt English with television as it doesn't seem that she goes to school or plays with other children.
I hope that long explanation helps!