Ring Age UK, Social Services and ask them to do an assessment and speak to the GP. Make absolutely sure you tell all these three groups that your Grandfather is becoming violent towards his wife. This should light all the blue touch papers and you should get an immediate response from all of them.
When you talk about your nana's behaviour, you do not explain what this behaviour is. Is she becoming less with it mentally? Not remembering what is said to her or not understanding, forgetting to do things; turn the gas off or lock the front door for example or doing irrational things; constantly turning all the lights off or something like that. If that is the case she may be developing dementia. If your nana's behaviour is keeping your grandfather awake at night, part of his problems may arise from sleep-deprivation or because he is exhausted coping with her behaviour.
What ever the problems, you need to seek help as soon as possible, start with Social Services and GP and then Age UK (used to be Age Concern).
It may be that they need carers in. Get Social Services in to organise . They will do an assessment of their income and savings (not the value of their house) to calculate how much they pay. They should qualify for a disability benefit for the elderly known as Attendance Allowance. The value of this varies according to their needs and income but could be worth between £50 and £80 a week EACH and if their income is low, even more
Keep us posted, I am the first (I think) of many Gransnetters who can help and advise you, so keep us posted.