I thought that giving a bad reference is illegal nowdays.All an ex employer needs to disclose is start and finish dates and nothing else.Character references are obtained from pople that one specifies.Besides, if you were working towards your licence and then resigned in order to build experience with another company the ex boss will be a bit upset dont you think.
And if a new company takes the word of and ex employer to make a judgement on ones character, than that says a lot about them.