Front Brake Problem

sonnyb

I have a Boardman Urban Pro with Avid hydraulic brakes. Last week I had a squeaking wheel which I cured by applying some oil around the axle/spindle. However since that time the front brake has lost all its effectiveness. It has lost the excellent braking power it had and I cannot think why. The oil did not touch the disc and I cleaned it anyway. Today I took it back to Halfords and they adjusted the brake and cleaned off the disc again but it is the same. It just feels weak and spongy and has simply lost its stopping power. I am baffled. Does anyone have any ideas please?