This is very messy, and you're apparently in court today, with your parent's solicitor, but not with your own solicitor, whom you are not going to contact until next week.
It appears that somehow your husband signed a payment agreement, (or his father signed it, either with authority as he's part of a partnership with your husband, or he forged your husband's signature). Right now you're trying to prove that your husband was not a party to the agreement, but there is the issue of the partnership. And if you say the father falsyfied documents then the father could be in trouble for that. But I guess if the father has already lost his home, he's nothing much else left and the judge may go easy on him.
It's a very stressful situation for everybody, cannot imagine what it must be like for your parent's in law, having already lost their home (not sure if they've lost the farm too). But put your best foot forward today, be as honest as you can, if you don't have the means to pay the bill, should you be liable for same, then the judge cannot order you to pay it. I presume you have no assets. In the interests of family relationships it would be best if everybody got along, pulled together, and didn't start the blame game, it will have no winners.