i have a w plate focus 1.8lx owned for bout 3weeks or so last week it started losin power but only if tank is less than quarter full ive had it on diagnostic machine showing no problems changed leads and plugs also air filter n gunna do fuel filter also no warning lights showing can anyone help?

have you noticed there is an audible "hiss" when you undo the fuel cap ???one of the possible causes of fuel starvation is a blocked vent hose to the petrol tank...try removing the fuel cap(or use one of the emergency caps for a while) to see if things improve.
http://www.cableuniverse.co.uk/emergency-fuel-cap.html
worth a try just to rule it out.