You're not going to find a decent plasma for £500 or less. Use pricerunner.co.uk to get the current market prices for the good brands. Remember that you can't actually get a plasma under 37" in size, and most are bigger. If you want a small flatscreen it will have to be LCD.
I'm waiting for the Panny 50" plasma, but due to my CRT going bang I had to find a Temp T.V.
Following a number of good reviews, I went for a Hannspree 37" LCD from Intekx for under £500 delivered with 2 yrs warranty. Teleland is now offering the same T.V with 5yr warranty for £450.
Worth considering in my opinion as I'm really happy with it.