The Polo BlueMotion is exactly the car you need, but it's too expensive for you.
Personally I'd look at the Skoda Fabia TDI. One like this: http://www.autotrader.co.uk/classified/advert/201049374946939/sort/priceasc/usedcars/maximum-mileage/up_to_40000_miles/price-to/5000/price-from/4000/fuel-type/diesel/model/fabia/make/skoda/page/1/radius/1501/postcode/pe26ea?logcode=p
Have you considered leasing an electric car? £397 per month gets you a Nissan Leaf on a three year PCP deal, only cost a few quid to 'fill up' and will do 100 miles to a charge. More than enough for what you need. Also, congestion charge exempt and free road tax as well.