Changed the oil today. All was fine.

Changed the washer on the inside of the sump plug bolt as well. Tightened it up good and proper, but now it leaks. Small drops at a time, but it leaves a decent few drops on the ground after a while. The bolt is not stripped and wasn't/isn't over-tightened.

When it happened on my old motor, I eventually took it to a mechanic, who said it wasn't tight enough. I know thats not true. Surely there's a reason why it's now leaking? It wasn't before. And I really don't want to drain the fresh oil out to fix it. Especially as I'm out all tomorrow and driving 2 hours to go away next week