Question all: I'm currently working my way up in weight ranges of 2.5kg, when I start to get some resistance (ie I can;t complete the 3x8 sets) shall I drop the weight down again one or two notches? Or what?
No, weight training is meant to be hard!
Stick at it until you hit (or surpass) your rep target and then increase weight next time. Sometimes it might take you a few weaks to increase weight but it'll happen. You'll probably get those extra 2 reps next week.
As for the shoulder pain you mentioned, you should probably get to the root of that. Shoulders can be a bitch.