I just feel really selfish because I'm worried about how much it will cost. Mum has no savings or property and her only income in Income Support and DLA. I don't know if I can afford to pay for home fees.
You'll find there's a lot of background information on the Society's factsheets which will reassure you about this:
Paying care home fees
When does the local authority pay for care?
If you are at all unsure, then perhaps you could talk it over with someone from you local Alzheimers Society branch: If you type your postcode into the box on the RHS then you'll get the local contact information. They will be able to give you advice specific to your area.
There's loads of help out there and we will help you find it. You and your mum are not alone