Home blood pressure readings are really useful; not sure how able your gran is, but I guess family members can help out (and check own BP) if needed.
You want a device that goes on the upper arm (not on wrist) and approved by British Hypertension Society:-
http://bhsoc.org/bp-monitors/bp-monitors/for-home-use/
Some meters are labelled "Clinically validated" but I am not sure what that means and would not trust it.
One would measure BP 3 times at intervals of a few minutes, record all 3 results, but in reality, it is just the last reading that one would use, and if aged over 55, then the top, systolic number gives all the info needed in clinical practice
best wishes