derbi senda 54 plate leak?

i own a derbi senda on a 54 plate and it started to leak but i tightened all the bolts and put a piece of paper towel under it to check if it was still leak leaking waited for half an hour and the towell was still dry so i thought i'd fixed it but i came back to it today and there is a considerable amount on the towel and i've had to cancel my mot that was booked for tomorow can anybody help me out i've checked everything and it seems to be coming from the casing next to the back brake lever and collecting on the sump plug before making a mess