Second run with my "free" Camelback (magazine sub) and much more successful from the off.
First tip I found out myself: Camelbacks are not rucksacks. If you are used to using long distance, well adjusted sacks perhaps as in backpacking/long distance walking, forget that for this type of sack, or mine anyway. The fitting of this type of sack, without a waist belt, needs to be adjusted so that it doesn't sit close and tight. Probably best sorted on the run in a 'suck it and see' sort of way. On my first run I spent the first couple of k sorting it out.
Second tip I found on t'internet. And it is probably obvious to many but didn't occur to me. Put water into the bladder. Tighten the stopper, turn upside down and suck on the mouthpiece until the water flows to get rid of the air. This stops you being chased by the "Sloshers". I spent my first run with the sound of water sloshing around behind me - kept thinking "if this was milk I'd have butter by the time I get home"😀.
I do like the Camelback and will wear it for my longer/more remote runs.