Just don't park anywhere where parking wardens are about often. As they will just see that you have no tax disk in the window and clamp you without thinking about it. You then get a clamp and an A4 sized sticker on you window saying untaxed vehicle.
I had it happen to me whilst my tax disk was in the post, I rang the number to get the clamp removed but they wouldn't do so without proof that the vehicle had been taxed. So I the had to do alot of running about to get the DVLA to call the clamping firm to tell them to remove it.
Which took 2 days. I was almost at the point of punching some one I was getting that peed off, and my car was'nt in the best part of town either. I was close to cutting the bloody thing off myself.
Gav