Hi, This is a very common fault. Im a vauxhall tech. As above it comes in through the brake servo seal, it also comes in through the body control module box. This is positioned on the pasenger side next to the battery. You will see it under the scuttle panel. This box houses the BCM and wireing loom that enters the inside of the car. This is an easy fix. you have to buy a new lid as it comes with a new seal on it. However to fit you need to take the battery and scuttle panel out. Where it comes in through the brake servo you need to strip all the scuttle, wiper linkage, brake servo and pollon filter out to gain acses, you also need to disconect the slave cylinder feed pipe so this will need bleeding after. you have to scrape all the old sealant out clean down and replace with a nice bead of sealant.
Any questions just ask me. Ive got to do one next week so i can take some pictures for you if you like.