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 Out Tongue :p :p](/styles/default/xenforo/vbSmilies/Normal/tongue.gif)
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.