fido and Brian might be on the right track. Check the feed pipe connection at the tank, the oil filter/drain joint - and check to see if the tank is leaking; such a leak is not usual but it does happen.
Try this technique for cleaning:
Fit a Hose End Sprayer (Gilmour, US$3 at Wal-Mart, or equivalent) to your garden hose and fill it with a 50/50 mix of water and Simple Green or other biodegradable liquid soap. Connect the hose to hot and cold water taps using a Y-connector, and hose the area with the hottest water you can. The combination of hot water, soap and water velocity does a marvelous job of cleaning. And the area tends to dry quickly afterward. Your laundry room is probably the best source for hot and cold water taps.
Keep us posted, Simon.