Help needed with Vauxhall (Opel) Corsa oil leak

Hi,

i have a british vauxhall (opel) corsa 1.6i 16v sport (v reg 1999), 60,000 miles, manual gears.

in november '06 I started to hear a tapping noise coming from the engine, so I checked the oil. The oil on the dipstick was light tan in colour and was well below the normal level, it almost looked empty. I took it to a well known garage (not naming any names) for advice on the tapping noise and for an oil change. After changing the oil, the mechanic told me that it didn't really need to be changed, and that the tapping noise wasn't related to the oil level. However, when I drove away from the garage, the tapping had completely stopped.
it is now june '07 and the tapping noise has started again. I am yet to check the oil level, but I am guessing it is going to be low. I have noticed that where I park my car, the is an oil patch on the ground, and it isn't just a few drops - it is fairly large. I can only assume that engine oil has been slowly leaking for some time now.
i am putting the car in for another oil change, as I am driving from newcastle to blackpool and back again next week, so I need to get it topped up. My only questions are, why is oil leaking in the first place, what is it that needs repaired and how much does it typically cost? Oh, and how often should I normally need to change the oil, if there are no problems/leaks?

thanks for your help.

s. Cooper