hello! I have got myself an iPhone4, is it possible to transfer all my old numbers and photos etc across from the old 3GS somehow in one go?

Yes its very easy.

Plug your old 3GS into your computer and on iTunes choose to do a backup.

Once the backup is finished plug your new iPhone 4 into your computer and iTunes will ask you if you want to restore from your last back up? Choose yes and this will transfer all your info, including texts etc., onto your new phone.

In ignorance I had previously sync'd the new phone as a new handset while it had the PAYG sim in, so now it won't back up from the other handset's info! Is there any way round this??

Do a device restore in iTunes on the new iPhone. It will reinstall iOS on the phone and then ask you if you want to set it up as a new phone or restore from a backup. Choose restore from backup and select the backup from your old iPhone.

thank you! do i need to save any pictures etc before i do this? will restore wipe old info off

well i did do it but all the apps i had have gone ... oh well, back to app store!

It will recognise that you had already purchased them so it will be free to re-install them.