i banked a cheque for 6.6k this week but ive just noticed the bank have credited my account with 66k!!

obviously im not going to spend it but im not telling them about it, was just going to let them find their mistake and reap the interest on the extra 60k that aint mine

different bank once credited me with £150 for no reason and i spent that without knowing about it, when they wrote to me asking for it back i told them to piss off and that id pay it back at £5 a month like it or lump it, they accepted

where do i stand?