Black cod as a "takoyaki type food"???
If as Ibrahim.Salha suggests you mean Izakaya foods then yes, Asakusa is one of the cheaper options and the food extremely authentic. However, I'm also not a fan of their takoyaki. The quality of the black cod will vary according to how they sourced it, it can be very nice here, but not always. At Asakusa I tend to go for a local fish rather than something like sanma/saury which has been flown over.
Takoyaki is an odd dish to open your query with as it conjures up (to me) images of takoyaki stalls which specialise exclusively in this food, outside busy stations, at festivals and in department store food halls.
I've seen a takoyaki stall of this kind outside the Bloomsbury Waitrose on the weekend days when there is a food market there. It was OK, not something I'd have again.
The best takoyaki I've had in London was at Aki - the Japanese 'bistro' on Grays Inn Road. I guess I rate Aki as being better quality than Asakusa overall but this is reflected in the prices. Doesn't mean the takoyaki was great, just about the quality I would expect from a Japanese place that doesn't exclusively sell this speciality.
If you aren't hung up on takoyaki and just want to find something that approximates a decent izakaya there's Tajima Tei near Farringdon station. It has more of a leaning towards sushi and sashimi (which it does very well) but there are other grilled and breaded options including the grilled black cod in miso you're after. (Takoyaki is probably too lowly for this joint.) Expect to pay around £30 and up here if you come in the evening - it's about twice the price of Asakusa.
Thank you both for your replies. And yes, I did mean izakaya. I'm American-Canadian, and izakaya is quite a new concept for us in N. America. And, of course, our interpretation is typically mish-mashed and not true-to-type. Hence, for example, takoyaki is a central dish at most izakaya-style places. Montréal's first pretty-close-to-being-authentic izakaya place is only a little more than a year old, and one of my students, knowing I like to try different food, took me there. "It's pretty correct" he said, "I can't wait to get some more takoyaki—haven't had any since I left Tokyo."
Different experiences on either side of the pond.
Tnx so much for your recommendations!
In that case, also, there's Jin Kichi in Hampstead.
No takoyaki (at least not on the menu as far as I can remember) but it does have the black cod in miso. Jin kichi's forte (IMO) is the yakitori - grilled chicken on skewers - and yakimono (grilled things not necessarily chicken - there's a place in Camden that says they offer yakitori beef, literally grilled chicken beef)