Yup -- you're misssing a very basic item: AE doesn't work like Premiere Pro. It's not anything like PP, and it doesn't behave like it.
One way is the method you need to play video. Space bars just don't work. You need to do a RAM Preview -- hit the zero key on the numeric keypad, let AE render the frames it can, and then it plays them back as fast as it can.
Dave is right.
As someone new to AE, you should really start here: Getting started with After Effects
The spacebar playback thing isn't the only thing that'll bite you in the butt and the resources I've linked to will help to prevent you getting bitten again.
Thank you both. This is actually reassuring, I would rather have zero faith in myself than in my technolgy! I have also mentioned AE in a post I've made in the PP forum, I'd of course love your advice there but please just refer me back here for all the AE related bits. Thanks. Tony