They are using an Aussie actor who can't be bothered to maintain a 'cockney' accent even though he is an Inspector from the Yard!
Series 4 episode 20 go watch it on your streaming video service of choice. It's called 'The Limey' for the love of Pete - why! Do they think we are too stupid to notice that Inspector Knacker is actually an Oz?
He says lines like "yes [Sheila] I grew up in the East end, then got a [fare dinkum] scholarship for Eton"
honestly he might as well be wearing a hat with little dangling corks and necking an ice cold pint of fosters. Ffs!
Sorry I had to vent but I quite enjoy Castle usually and this just boiled my poss - I feel better now though cobbers