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.
This doesn't sound like a good idea at all.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?