Mattress advice for elderly mother

I am looking for a single mattress for my elderly mother who suffers back, hip and knee pain. She currently has a memory foam topped, pocket sprung mattress that is around 5 yrs old and she feels it has lost it's body and complains of pain every morning!

She has a small budget of around £300 and I have seen a pocket sprung mattress with good reviews made by the British Bed Company. They seem tone related to Mattressman website and I am a little suspicious. What are your views and can you suggest something suitable for my mother, please?

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