autocheck.co.uk
There are more out there, but this is currently the easiest for me (it doesn't ask for you to type the silly images in).
Then if that is valid you can check it against askmid.com (if you are the owner, remember this can be up to 7 days out of date, so a car with no insurance might just have insurance... check again in 7 days).
Then you can do a check with direct.gov which will show you if its "licenced" (car tax).
http://www.taxdisc.direct.gov.uk/EvlPortalApp/" vehicle enquiry.