Just a suggestion but have you checked the fuse hasn't gone? I tried changing some interior lights a couple of months back and the following morning went to drive off and my steering weighed a ton (also aircon and horn strangely) changed one fuse (forget which one, sorry) and hey presto all working fine again. My A4 09 sline back to light as feather. I had Halfords telling me it was gonna be the pump.