If your mum has the money in her bank, I suggest you contact the company/companies the £347 is for and explain the situation. Just tell them your mum has passed away and you’re left to pay this bill, but would they mind putting it off for a month or 2 whilst you’re grieving and sorting out the funeral. Depending on the company, they may see it as bereavement rather than you not paying and be lenient with you. If you’re going to use the money, make sure you contact them first.
Also, did your mum leave you that money, or did she leave it in the bank just for bills? I’m no lawyer but if you fail to repay the debt she left the money for, you could get in shit for it because you took it without permission (a will or whatever she left as proof).