Bit embarrassing - just attached the Maxi Cosi Pebble car seat to the Uppababy frame for the first time...all very easy and very simple. Then thought I should probably try to remove it too and, errrr, I can't 
The instructions say to "Squeeze release on back of car seat and lift" - Can't for the life of me find a release on the back of the car seat!!
Anyone out there able to help?!