Replace the seal and use a little gasket maker around the edges. If you can afford it buy one of the remote filter kits. Much easier to change the oil. Leaking British cars are a sign of careless work.
If you are cranking the bolts down REALLY hard, you are killing the gasket... Are you using a good gasket sealer?? I use permatex aircraft sealer.. (comes in a little jar.. stuff is nasty brown) follow the instructions and use a new gasket.. If it's not cracked then this stuff should fix it.. I have never had a leak yet..