Solved my problem of ejecting rear bottles! :)
I have lost two bottles now from the rear cage, which is annoying!
The problem was that the Sitaero saddle mounts for the bottle cage sit at around 30 degrees of horizontal. The bottle just want to fly out.
I tried all sorts of things to bring the cage more vertical and finally I think I have the solution and its a very simple one.
one long bolt and a bit of plastic pipe (actually a big rawl plug)
tried and tested during the BRAT tri and it worked a treat, the rear bottle hadn’t moved an inch
so now my front and rear hydration is sorted, plenty of capacity for Olympic distance races