I had an old astra and had great problems with car seats.
I had a britax club class in mine suitable from birth to 4 as it fits in both directions.
This has been superseded by the first class-I think.
Try the britax web site to see what would fit and then go to somewhere that specialses in baby equipment and get it properly fitted.
I got mine done at a big mothercare and they were fab.
Where in the country are you?
We've got an Astra had we have a maxi cosi priori stage 2 seat. Got it checked a month or so ago and we have buckle crunch. Very common on Astras apparantly as the seatbelts clips are longer than most.
ja9 - just thought I'd say we have two car seats (0 - 4ones). One is brand new made by vauxhall and one is Britax rockatot.
The vauxhall one is much heavier than the other so we ended up using the britax one and the vauxhall one remains unused!!!
I will be putting these up for sale on ebay at somestage but let me know if you want any details or if interested - its obviously specially made for a vauxhall car!