I would advise against saying anything negative about your former employer. You don't know what he said about you ~ negatively or positively. If he's that big of a jerk, they probably are wise to him. I would have some concerns about hiring someone who spoke negatively about their former employer. It would make me wonder what they would be saying about me when they had moved on.

I think show interest in working "as needed" for a while. However, re-state that your intention is a full-time position. And, that you intend to keep working towards that goal. Hopefully, with them, but if not, you will be investigating other opportunities.

It could be that they really want you, but are concerned about some negative things your previous boss MAY have said (assume he's innocent at this point). Once you accept an assignment from them, you will prove yourself and they will realize that whatever your old boss said, needs to be taken with a grain of salt.

Unfortunately, they may not be able to guarantee you enough hours to make it worth your while and you will have to move on. One thing you might offer to do is to accept 1 assignment free of charge so that you can show them what you are capable of.

This worked for me years ago when I was desperate for a job but did not have the required skills yet. I offered to work for one week without pay to show them that I was capable and eager to learn. That one week of missed pay was well worth the 15 years of employment it led to!

Lisa