To get a comparison on costs, you could try the following Website.
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http://www.broadbandspeedchecker.co.uk/# >
On the right side of the page is a window, in which it can tell you the best possible speeds you can expect. In the dropdown box pick the Essentials 24MB -40GB limit and then enter your mother's postcode in the box underneath. You will get a selection of providers with relevant costs and the best expected speeds available. I also tells you exactly how far she is from the local exchange. This can have a significant bearing on what speeds can be provided.
Hope this helps, whichever provider you go for.