In my opinion, I wouldn't mess around with the dosages too much. If you can't measure exactly, you don't know how much you're getting, and a few days of a lower dose isn't going to make a massive difference. I'd just stick with the 20mg, and then call immediately on Monday morning to have it rectified.

would you suggest taking 20mg one day, followed by 40mg the next, then alternating like that until I run out and can then request the correct 30mg dose from the gp? Would this work?
This doesn't sound like a good idea at all.