One reply I had from a previous question I asked on this matter was that the garage may not have added any sealer at all and may have temporarily stopped the oil leak by using a thicker (higher viscosity) gearbox oil than Ford recommend so the leak would be less noticeable.
The oil leak did seem to disappear completely for 3 - 4 days after the garage used a sealer in the gearbox oil but then when I put cardboard underneath the car when it was parked outside my house there were some drops of oil on it again.
Do you think it was just a case of the sealer in the oil only working very temporarily rather than them using a thicker oil to stem the leak ?
thanksSolution to Ford Focus manual gearbox oil leak?
Try replacing the seals.