Answer

The spring constant is approx. 25 N/m.
 The figure I actually got when measuring was 24.53 N/m but the values will probably vary slightly from spring to spring, so 25 N/m is a good approximation.
 Larger mass increases (e.g. 100g increases) give more reliable results for these springs.

 This was calculated as:
 Spring constant: k = F/x
 Where:
 F = Force in Newtons, i.e. mass in kg x ‘g’ (9.81)
 k = Spring constant (N/m)
 x = Distance or amount spring has been stretched (final length minus starting length) in metres

 See also Word doc 'Hooke’s law and spring constant for SPR-300-100'