Thank you all for your comments. Have decided not to go down the dog route tempting as it is.
OH seems to be declining quite rapidly - last Sept he had 26 out of 30 in memory test and last week it was 19 out of 30. Will try and search out other things for him to get interested in as if I really think about it how will I cope with him and a dog in a year or so if I cannot leave the house for long? Also, I can see him giving too much food to the dog and it being a bone [excuse the pun] of contention.
I had to rehome my mums dog when she went into hospital and then care home. That was really stressful (i work long hours and have cats so taking him on wasnt an option) but i found him a lovely home with a wonderful lady - he has the life of riley and looks younger and more fit than ever. (I think mum used to feed him inappropriately) i had a dog walker take him out every day but he gets much more exercise now.
I actually got her a cuddly golden spaniel toy from that big toy warehouse. It looked quite lifelike. She loved it for about a year and carried it everywhere with her, looking after it. I even bought spares in case, heaven forbid, it went walkabout (which does happen in care homes!!) as she would have been very upset. Then she lost interest in it. But it gave her a lot of pleasure in that period - i guess it depends where on their dementia journey they are.