Yeah, but restaurants do tend to do this, look at Mickey D's and Burger King. Where I live, there are literally next door to each other with just a fence and some parking spaces separating them.
I can appreciate this, but it's neither in the best interest of the consumer or the business, in my opinion. If they opened in Angel/Islington because of a huge lunch crowd/busy evening street, then there's loads of places around London that also easily meet those criteria. It's quite obvious they've opened there to be in direct competition with Mucho Mas and for little other reason. I can appreciate that the niche Mexican-food market might be seen as a 'risk' and they wanted a 'proven' crowd, but I personally feel it's a bit distasteful to not only have almost identical menus to an already-established place but to pretty much copy the décor as well - the bench-style seating was at the Mucho Mas premises far before the current owners (I'm not saying it's a totally original style by any means, of course - but far too coincidental for Tortilla to now have the same).
I haven't personally visited Tortilla, I give this disclaimer, but I did have a nose around their website. Maybe I'm just a bit emotionally attached in that I'd hate to see a place a nice as Mucho Mas go under just because this other place has shown up with a clearly larger budget to woo the indiscriminate North London working crowd. I can appreciate this could be seen as 'good business' on Tortilla's part, but that doesn't mean I like it. :-/