If she likes jewellery how about a nice item of jewellery, and you could perhaps get it personalised. A nice locket maybe and pop in your photo and grandchildren (if any).
Or you could sort out a pampering / makeover day if she likes to get glammed up. If you do a search on the internet I think there are some studios across the UK which sell such days as vouchers.
Thanks everyone for all your brilliant answers!!! will share them with my dad and see what we come up with, I have found something on buyagift, a massage, followed by makeover and professional photos done...think im gonna do that for her, and pass all your comments onto me dad and let him take his pick! Thanks again!!! x
make a scratch book, in which you can post some of your pictures with your mum, some sweet moments/details you could recall, some souvenir she might have forgotten...
i am sure this sentimental gift would make her happy.:)