I had to stop a couple of times on the way home to move the seat up, and no matter how tightly I did the clamp, the post would always slip down after a few hundred metres.
I removed the seat post and cleaned all the grease off, convinced that this would prevent the problem from happening - but it didn't. Eventually I got so annoyed that I took the bike back to the shop and got them to replace the clamp with something more robust. The other thing I did was to follow the advice of a few people on YouTube and place a piece of gaffer tape around the post where it disappears into the frame. This way you can see at a glance whether your seat has slipped down into the frame, and where the original position was.
Since fitting this new clamp, the seat has not moved.