Can they demand this information?

I took out a loan a couple of years ago in my name when I was working. When I lost my job, I agreed to re-pay the loan at a lower rate because it was only my husbands wage coming in. Recently, I missed one payment and was then bombarded with phone calls and letters. They now want to increase the payment (my husband does not know of the loan) and have sent an expenditure form asking for all my husbands work details and earnings. I am still paying the lower payment, but because of this one missed payment, they are demanding this information. I do not receive any benefits. If the loan is in my name only, can they threaten me for these details? Any advice welcome please.