I stumbled upon this place after reading the reviews here... and then had to try it out for myself. I've been to just about every Mexican restaurant in London (became hooked after spending a year in the US) and have been to Mucho Mas about 5 times now. It seems like a few of below reviews are from Americans - which is all well and good, but we're in London, so here's my English perspective...
My last visit was Wednesday evening - the place was completely full on both ground and first floors. I had a steak burrito with pinto beans (which are a new menu addition - has only been black beans until now). Steak was absolutely lovely as always - super juicy, tender, tangy with a hint of spice. The pinto beans had a nice meaty flavour - spoke with one of staff and apparently they're made with a bit of bacon. Rest of the burrito was extremely fresh tasting - which is why I keep coming back. From my first visit I've been shocked with the quality and quantity of food you get for the price.
Washed everything down with a Modelo Negra, or Negra Modelo - can't remember exact name but it's a dark Mexican beer - very tasty as well.
As for the sour cream (read a couple of comments about it below) - I love it. Apparently they put a bit of coriander and lime juice in it... to liven it up a bit.