On further inspection, if appears that in addition to the bent arm there is a slight burr on the plastic where I assume it was released from the mould. looking at where the burr is, it will have been this that was rubbing on the inside of the runner next to the spring and causing the stickiness. A gentle rub with fine sandpaper would sort that out nicely. It was only one valve that stuck - the other two were fine. Judging by the performance of the other two, they are fine for my cornet (a Chambourd 106XLR). I must have just been unlucky!
Bumped the review back to 4 stars as Dawkes service was great and the guides, although very slightly different to the ones that came out, appear to be the right ones after all...
