Right, thanks to all for the very informative, constructive and educational comments.
I liked the bruteforce approach and agree that often the simple ways are the best, on the otherhand I was learning a lot fiddling around with some of the more smoke and mirror approaches so I went that route.
So, heres what Ive actually done - with my sincere thanks to coyetemax who has been working away in the background to guide me in the right direction as well as being a useful sounding board for some other ideas.
I was looking for a red/white dashed border ( although Im now wondering if black might be more suitable - thats another story! ).
First I created the rectanglar border on a seperate layer called "border" thus:-
Created a rectangular selection
fill red
contract 1 pixel
fill white
contract 4 pixels
file red
contract 1 pixel
delete
so I now have a 4 pixel wide border around my world map, filled with white and edged with red.
[the procedure from this point forward is credited to coyetemax ]
Create and select a layer called "grid"
created the required "grid" using the single column/row marquee tools ( in "add to" mode so the whole grid is eventually selected )
Fill the selection with red and deselect.
So I now have a double red border on one layer which is in effect intersected by the single pixel wide red grid on the other layer.
Now have to remove the superflous lines ( ie anything except those lines within the double border )
Select the border layer and using the magic wand select everything on the outer side and inner side ( ie where the map goes ) of the border.
select the grid layer
Hit delete
Merge the two layers into the "border" layer ( so the grid layer goes ) to leave a red border, filled with white and broken into dashes by red lines.
Then simply hop around and bucket fill the appropriate squares with red.
Job done.
Looks long winded when you write it down but in reality its quick, simple and works a treat.
Frankly had I been left to my own devices rather than guided in the right direction I would have likely have come up with the brute force approach reasonably quickly but learnt little that I didnt already know. There is nothing wrong with the brute force way and it will work - however being patiently guided through this layered approach, not spoonfed but rather pointed in the right direction - left to some trial and error but being helped when necessary means that my knowledge has over the last few days increased tremendously. More importantly Ive started to think in the right way to build up an image from scratch rather than amending existing images which is what I am used to.
More than that, whilst this was to be a "one off" image Ive actually been inspired to make a few more maps ( when I finish this one! ) - Ive seen potential for the techniques Ive found here to create a 3 dimensional map of my village ( a very old village in the hills of central England and famous as the home of Florence Nightingale ). The local parish council were looking for some new tourist boards - and Ive been inspired!
Thank you!!