Unless you have a large number of posts to set I would strongly suggest using bagged premixed concrete. In the states we have several different brands, all of which do a good job.
If you are not going to put weight on your posts for a couple of days, I have had great success with using the product above dry.
Wet the hole a bit then insert pole and install whatever steadying/alignment aids you feel neccessary. (Use more of these than you think you need) Now dump in the premixed concrete and use a tamping devive to pack the hole tightly.
Usually I would install all of the posts in this manner before I used the water hose to soak the top of the concrete in each hole and the ground around it. You need not try to mix the water with the concrete. It will draw water from the ground , which you have supplied, and be solid enough for further weight after a dy or two at most.
I have used this dry method many times and have been more than satisfied, and you will find it much easier mixing and moving wet concrete.