Things to know before you go;
As mentioned, take some drink/food if you intend to be a while, the location is quite a way from any shops.
Don't leave valuables on show in the car, visitors to the "old" cemetery have to park in a small layby alongside the main road and you will be far from your car at times and unable to see it clearly.
The newer and St Swithins area has off road parking but access to the burial areas is again quite a distance from the parking area and you cannot see your vehicle, nor can passers by.
There are toilets but they are situated in the furthest corner from the main road.
The soil is quite sandy and the area is a favourite with rabbits, beware stumbling into a rabbit hole and twisting an ankle or similiar.
Until you see the layout of the cemetery "in person" it is difficult to visualise the size and layout, the newer section has an easier to follow rectangular pattern, the older section with the curved sections can be a real pain. Try to be methodical, it will save hours of time and miles of walking.
A worthwhile exercise if you can do so is to visit the library and check the Monumental Inscription listings.
Study the surname index for your surnames, for example you might find the surname Willis listed at A10, A15, B23, C103 etc
A map showing which section is "A", "B" etc is in fact the cover for the monumental listings, either photocopy the plans or ask the staff to do it for you (if memory serves it is 60 pence per copy) but it will help you find your bearings when you visit.
It's a beautiful place but there is very little shelter from the elements so take precautions against the extremes of the weather