Thanks Germanshepherdsmum, but now it's 'dead' she won't agree to do anything with it. I am going to suggest she give it to her neighbour, whose parents are very happy with theirs!
I've bought her a Nokia 130 with big buttons and a small screen, I think she'd prefer it to look more like a 'phone'. I was going round in circles trying to find the Vodaphone PAYG 1 deal where you pay £1 per day for use, and came across the 1pmobile SIM service. It's a triple SIM so should fit the phone, and I've paid £36 and it should last her for the year. It was the top up texts I was trying to avoid.
I'm having it sent to me so I can set it up before I send it, but dread posting it on - she does become so unpleasant when she cannot understand something and the 'thing' is stupid.