how can i fix it??

my x reg golf started using extra petrol about the same time as the cat went,i got the cat replaced and put in 2 new probes front and back of the cat,i drove it for about 6 miles and for the first time the engine management light came on,ive had it on computers and keeps saying problem bank 1 sensor on cat, got the cat chequed at an mot centre and it passed, also put new plugs and leads on it??it is still using extra petrol?? can anyone help??