Hi, this insect was about an inch/inch and a half long. Didn't get too much of a look at it as it was on my car windscreen, but the back end of its body was flat and half yellow, half black. It seemed to have a short sting and it was a flying insect. Very slender not fat like a wasp although I thought wasp when I first saw it but cannot find anything on the net that looks like it. I live in Lancashire.

Hello,

You may have seen an ichneumon wasp (or fly).  These are related to wasps and bees, but have a very slender body.  They do not sting, but use the long slender 'needle' at the rear end to lay eggs in wood.  Some are actually parasitic laying an egg in another insect.  These are very important natural pest control organisms, which can keep pests species (like aphids) in check.  One of the problems with agricultural methods that we use today, is that insecticides are not species specific, but also kill the natural predators of pest species. 
Parasitic wasps of this kind also posed a big problem for Darwin when he was thinking about natural selection and evolution.  He could not understand why a benevolent God would have created these wasps, which lay their eggs in the bodies of a host, eating the still living host as they grow.

I have posted a link below to a wikipedia page on ichneumon wasps/flies.  I hope that this has answered your question. 

http://en.wikipedia.org/wiki/Ichneumon_wasp