Andy,
I guess you've looked here,
http://www.guitarforbeginners.com/barre.html.
If so and you're still having problems it could be that you just need to strengthen your barre-ing hand.
Alternatively, you don't need to play the whole thing. eg, when playing F in the first position I invariably just hold 1st and 2nd string down with my index finger whilst the others form the E shape. It doesn't matter much to me in a 6 barre chord regarding the root note 'cos as we all know 6th and 1st string are same note but 2 octaves apart.
Hope this helps a little bit.
Iain