There are so many pieces of correspondence that you should have received by now. Did you run up the debt while living there and now you’ve moved out? If so, is it possible that other letters have been thrown away?
If you have other debts, I would seriously recommend checking your credit report and there may be other CCJs or proceedings you don’t know about.
Despite what some people online will tell you, there are very few ways to avoid paying debt, beyond bankruptcy or an IVA. They will catch up with you eventually, and even if they don’t the defaults and other results of debt will destroy your credit rating.
My DH had a large phone bill when we had been to America. He disputed it since there was some problem with it, I can’t quite remember. He was offered a reduced figure which he stupidly refused. Went to debt collection and they cocked something up so he was able to get the debt written off and had an apology and a small amount of compensation from them. All good right? Nope, five years later we applied for a mortgage and despite it being written off it was still listed as a default on his file and the mortgage companies mostly wouldn’t touch us. One said they would if he paid off the debt, but since the company had written off the debt they wouldn’t take the money... it took months to resolve.
If you make a partial settlement that will show on your credit file too and make lenders reluctant anyway. Always pay debts if you can and if you can’t, always contact them and come to an arrangement.
If the creditor goes via the high court, you could have high court enforcement officers at your mums house taking anything that she can’t prove isn’t yours.