Restaurant: PittCue Co and Buen Provecho, both London SE1

Here's a trend: food carts. They're all over the place at the moment. These can be mobile vans or semi-mobile wheeled carts or non-mobile stalls, but they all share a provisional, temporary, ramshackle and crash-landed quality. The vogue seems to have begun with high-class burger vans, the best known of them being a cult favourite called the Meat Wagon, which roams the periphery of south London and updates fans on its whereabouts via Twitter. (The Meat Wagon is currently in a summer "residency" at the Rye pub in Peckham.) These carts make particular sense for certain types of fast food, where the cook can do most of the prep in advance and assemble the grub to order. I like this austerity-chic trend: if you're going to eat fast food for lunch, as millions of people do, why not eat something cooked by someone who really cares about their speciality and who wants to serve their customers personally?

So, this week, two carts, both of them in the area around Waterloo station, and one of them there only for the summer. That would be Pitt Cue Co, which I suppose you'd have to define as a pop-up cart. Being a pop-up, it also sometimes pops down, and the first lunchtime I went it was shut. They don't have a phone or website, so you can't check when they're open. I had a mild sense of humour failure about that.

Pitt Cue is a van parked directly under Hungerford Bridge; so directly under it that when a gust of wind got up, a graffiti pen fell off and landed on my foot. Their thing is barbecue. The choice at lunch was beef brisket or pulled pork (I know it's immature, but the term "pulled pork" never fails to make me giggle), served in a cardboard box with beans and coleslaw, at £7 a go. The quality of the barbecued meat is very high: the brisket was unfatty and the texture reflected long, careful cooking, but the pork was even better, with an astonishing, but not overpowering, smoky flavour. I found the beans and slaw a little sweet, but then they often are in proper barbecue – one of the reasons I don't love-love-love it. I've eaten at Jamie Oliver's Barbecoa, and I'd say the barbecue at Pitt Cue is twice as good at a third of the price.

Meat aside, the other cool thing about Pitt Cue is that it's licensed. This is rare in the cart world, and they make the most of it by serving a variety of beers and cocktails, among them something called a pickle back: a shot of bourbon, a shot of pickle juice to wash it down and, when they have them, some pork rinds. A fortifying combination.

Buen Provecho, meanwhile, is on Lower Marsh, on the other side of Waterloo, and it's a stall rather than a cart. The cook here is Arturo Ortega Rodriguez and his thing is Mexican street food; this is promising, since Mexico has one of the world's best street cuisines. He cooks a range of sauces – on the day I went, five meaty and three veggie – which you can have either on a soft flour tortilla or in a lunch box with red rice and refried beans. Homemade salsa and guacamole are there to add to taste, as are hard tortilla chips if you fancy a bit of crunch. The boxes are £6, the tacos £2.50 each or £6 for three.

This cooking stands by the quality of the sauces, and these were excellent. The star was a pork stew, marinated in orange juice and spices and a sauce paste called achiote that I'd not encountered before – a rich, dry and complex mix. Beef in a smoked chilli sauce had a nicely deep flavour, well set off by the magnificently gloopy refried beans. Two chicken dishes – a stew and a chorizo mix – were a tad less interesting, and I left wishing I'd ordered another portion of that amazing pork.

Be warned, though: this food is seriously messy, especially the tacos. Many of Buen Provecho's customers were people in suits, and I was worried for them – I was wearing shorts and a T-shirt, and ended up looking like an extra from Cannibal Apocalypse. Note that Buen Provecho serves until the food runs out, which I'm told is usually around 2-2.30pm; wet wipes might be an idea.

PittCue Co Under Hungerford Bridge, Belvedere Road, London SE1 8XX. Open Wed-Sun, 1-10pm (7pm Sun). Meal with drink, about £10 a head.