I was an extra in Hippy Hippy Shake, which stars Cillian Murphy and Sienna Miller. It might be released Christmas. It's set around the time of the Isle of Wight festival. An interesting experience. They arranged travel and food, so at least you don't end up spending your earnings on those.
I got into it by accident, I was at a market one day, and a nearby casting agent must have thought me sufficiently hippy.
They booked me for six days, but I think I only did 3 or 4. I had a pretty uncomfortable time, so decided to call it quits after a while.
It was a cast of hundreds, full of extremely shallow and irritating wannabe actors. Plus the hours were horrendous. I had to get up at 5am each morning, and didn't get home till 10pm. I was in a very old costume that was slightly too small, and smelled like it hadn't been aired for 30 years. We had to queue for the use of filthy mobile toilets (you can imagine what one might look like after 500 people had been through it). It was actually a little too close to being at a real festival! We were herded around like sheep. "You, over there", and that kind of speak. It wasn't AT ALL glamorous. Don't go there expecting that.
The money was good, the food was good, and it's a funny escapade to tell after a few drinks. I wouldn't do it again however.
Having said that, you may have an easier time of it on a smaller more intimate piece which isn't a period production. I'm sure not all stories are like mine! If your heart's set on it, go for it!