Don't need to take out wheel arch. Either go through the fog light blanking panel, 2 srews and clips or if a light is in there just crawl under and lever the pump out trying not to get too wet, a bit awkward but there's harder jobs on P38's! You can check if its the pump or supply by cross connecting the wires. Original pumps have colour coded sockets to match up the plugs on the end of the wires but replacements as Saint says are all the same without the colours. Mine were £6ish on the Bay of E and work fine. Careful how you lever them out of the bottle, I managed to break one - and sods law, it was a working one.