What do I do? I should have been paid about £200-250 this morning, but instead have been paid £569.36!

I suppose they could just ask me for it back when I go in to work, but if they don't what should I do. Would it be dishonest to keep it, even though they are a big company, and woudn't miss it?

Damn, the worst thing is, I want to buy a synth this week, and this would just about give me enough for it...... morals or synth.