Is it your JCM900? You can send it to Marshall for a full service and they can arrange courier both ways.
I do biasing myself now I bought a bias tool on ebay US and it's very easy to use with great instructions. Just don't do what I did accidentally and short the pin 3 & 4! You'll hear a very big bang and your amp will die... Thanks to Heavier than Hell I fixed it though t was just the HT fuse :)
