I got the BabaBing Deluxe Daytripper Paternity Satchel. Not used it yet but seems roomy enough for all the baby essentials as mentioned above plus stuff from my handbag (wallet, hand cream, keys, tissues, lip balm, Oyster card, phone, note book) which will retire as soon as I start using this one.
I've also got the free boots one but the handle doesn't extend wide enough for the double buggy so we will use it as our downstairs storage for changing and keep a stock of nappies, wipes and a couple of spare vests/baby grows to save having to go upstairs.
I use the free Boot one. Not very pretty to look at but it does the job. I always wonder what people put inside those enormous changing bags. 2 spare vests, nappies, wipes, muslins, 2 bibs, 2 bottles and ready to go.