0So, after folks advice, I’ve chosen to base my gravestones on standard medium and small bases with a little bit of modelling to depict a few ‘monuments’. I figure this gives me the benefit of stand alone pieces that can also provide cover as area terrain.
I’ve purchased 2 sets of gravestones: 
The first is Langley Models (A36) N Scale Graves and Tombstones. These are well detailed and finely cast model gravestones from the 17th-20th Centuries:
To this, I’ve added some Peter Pig 15mm gravestones. By contrast, these are thick and rudely cast comedy gravestones (one even has RIP Big Gun scratched on it!) :)
Using double-sided tape, I’ve mocked up some HO Scale railings I bought years ago in Holland, but have neglected to erect the Langley stones (you’ve got to love those small footstones – how long is a 15mm corpse?):
The termporary grave markers in the bottom left corner will be modelled to represent the scene below:
Will glue and paint over the next couple of days.