Thanks again to Piglet & others for your help. I thought I'd report back because I learnt a few things. I'm almost through fixing up the new homemade shelves. They're fixed through the small upstand at the back as I didn't want brackets or any large visible fixings. The screws will be hidden when the shelves are painted & crowded with photo frames. The small central line of holes were pilots I drilled to test the wall. Turns out that it isn't lath & plaster like I thought but solid grey blockwork. 
I wonder if Piglet or anyone else could comment on this? House was built in 1920s. Good god it was like drilling reinforced concrete, bastard hard. Even with a new masonry bit & drill on hammer action. The upside of this is I have 3 inch deep super solid fixings and a shelf you could tap dance on which is great. One handy tip I learnt along the way was to use a small neodymium magnet hanging on a thread to detect the studs (see Youtube). I shall remember that in future. Also, I never considered that the wall wouldn't be perfectly true, so there is the odd very small gap behind my batten but I shall fill this in later. You might think 10 screws per batten an overkill but the shelf is 6" deep.
Over all I'm dead chuffed. I haven't attempted anything like this before. In the future I might be more patient & buy some recessed keyhole plates to use instead but no shops had them in stock & I didn't want to wait.