Be Excellent To Each Other

I don't know if it's illegal or not but is it allowed to put up the price per unit 275%? I'm talking about my market stall here and we don't get our electricity direct. We get billed by the market rather than an electricity company. Is it accepted that they can just charge whatever they want?

Do you have some sort of agreement with the market under which you're allowed to set up your stall there? This may be on a "you set up here and it's subject to our Ts&Cs" basis rather than on a signed contract, but either way I'd expect there to be something. And that something could presumably cover costs?

In the US it's illegal to resell electricity, but not here. So, I think you're out of luck.
What are they charging, and how much are you needing for your market stall (what are you running with it)?

I'm now getting charged 11p per unit whereas before it was 4p per unit. The bill has worked out at £112 for the last quarter. What am I running 10 flourescent light tubes. Two 100 watt incandescent bulbs, a computer and monitor. A TV and a vcr. Occasionally a games console that I have to test. The computer is left running all the time but everything else gets switched off. Over the last quarter I've used around 1000 units.

I'd talk to your electricity supplier about this, or the CAB. I believe there is legislation restricting electricity resellers' ability to price gouge, which is usually used to stop landlords making a fast buck on the backs of their tenants.

It's definitely a question that demands proper legal advice, beyond what any of us can tell you on the forum.

Well, as a practical solution I'd suggest taking your own electricity with you.

Don't run 10 fluorescent lights, use LED or CFL spots where needed, and you'll get the lighting budget down under 50W.
Consider one of those little LCD TVs that they use for cars, they only take 10W or so (or run videos on a laptop)
Use an inverter to run the game console stuff and charge a laptop, should be another 50W.
Then if you have a 100W average budget, you'll need about 20AH of 12V battery for every hour of run time. 5 hours = 1 caravan battery.

Notice that you've now reduced your usage to less than 1 unit a day, which is now eminently affordable if you choose to keep using their mains.

I'd also suggest that there's not necessarily anything wrong with the new rate.
You were clearly on an industrial rate before. Market stalls have a comparatively low average consumption, but a fairly high peak load (200kW say, on the market day) which requires a big feed. That's a bad combination and you'd expect to have a costly monthly "connection charge" to go with the low unit price, or alternatively a higher than normal unit charge (like pay as you go).
I'd suspect something like the connection charge was being subsidised by the council before and they've caught on and stopped doing it, or the supply was shared with a commercial user and the arrangement has changed.