poss oil leak
 

i was helping my sis move house today and as i went to leave her house i saw a damp patch on her drive(it's slightly slopped) had a quick look under the car and could not see anything leaking.Just a slite damp patch at the back of the sump. i checked the oil level when i got home and it was fine. just had another look under the car and sump is still oily on the back side but there is no oil on the drive. the patch on my sis's drive was quite big! prob 12inch long by 5inch wide. put my had in it and it was quite sticky and smelt like melted tar more than oil.
Am i just being paranoied or should i be worried? If the sump gasket is leaking how much would it cost to have it sorted?
Cars a 99 turbo 2000 if thats any help. i had a oil leak from the rear main crank seal abut a year ago but thats got sorted by the guys at api. could it be the crank seal again?