Some ideas:

If mum loves gardening, Marks and Spencer sell enamel polka dot herb pots for £9.50, or get her a watering can or some short wellies. gardening gloves, a book on gardening/plants/flowers.

Or a new teapot or some new herbal tea flavours, chocolates/champagne. Or perhaps a book/CD/DVD you know she would enjoy.

Bits of jewellery is always nice and costume jewellery is not too expensive.
There is the 1001 little beauty miracles book for £3.50 from Marks and Spencer or a new make-up bag/handbag.