under extrusion
Image taken under a loupe , looks to me that my extruder on a mk3s is under extruding ?
looked at the help pages and seems to suggest a increase in the extruder multiplier.
Using the default settings for PLA prusament with a calibrated Z level using the 75x75 print. 
RE: under extrusion
How did you arrive at that conclusion ? You cant really tell about under extrusion from a first layer calibration.
Slight gaps between lines are more an indication that your live z is slightly too high and so could do with more squish.
Under and over extrusion are normally checked with printing something like a cube with no top layers and 1 perimeter. You then measure the wall thickness with callipers to see if the average meets the expected (0.45 for a normal default profile with a 0.4 nozzle). If not then you check the filaments real world diameter measured and averaged at several points and adjust the filament width setting to match. Then retest. Of course all this is after confirming that the extruders steps per mm is correct. Which it should be for a mk3. If that still shows as being out, you then adjust extrusion multiplier.
Basically -
Check and confirm Steps for mm on the extruder is correct
Accurise the filament diameter to match the real filament
Check the feed gears are clean of debris and that the tension is ok
Print a single wall cube - measure
Then extrusion multiplier.
RE: under extrusion
I didn't conclude it was certainly ' under extrusion' but went by the images on the help pages for comparison,
Plan to take the course of action you suggested after adjusting my Z level slightly down.
Thanks for helpful detailed response its appreciated.