You could hire a strimmer or brush cutter for the day, better still find someone on the site who has one and butter them up with beer/compliments etc ;D
Then, once you've strimmed it, cover everything to keep the grass weeds down while you dig a bit at a time. I was pretty skint when I took on my plot last year and didn't cover after I'd strimmed and just started digging. By the time I'd dug about half the plot it was autumn and the weeds went mental, I had to strim it again and re dig the bit I'd done first! :o
Good luck ;D