You'll do well to get a new diesel at anywhere near £5k.
Fiatsupsaver are usually a good bet, currently £5998. But they often seem to have special offers on Pandas, most recently £5895.
Nearly new ones seem like hens teeth. When I was looking 3m ago I could'nt find a single used one being advertised at below the price I could buy new, although with no trade in, I expect I could have haggled a bit.
Anyhow I wish you the best of luck. I reckon that at £6k this is probably the new car with the lowest overall running cost over 3yrs/36000m in the UK. ('What Car' reckons its a Toyota Aygo, but they did'nt allow for the substantially higher discounts available on a Panda!)