Thanks for the reply. Unfortunately, I'm already aware of all that, and still have slow burn speed. Just a summary:
The drive (the 825 matshita) supports dvd-r at 4x.
I've only ever used dvd-r 4x or higher rated media, including 8x and 16x rated disks.
I've only ever used high quality media, verbatim, maxell, ritech etc. I've used at least 5 different brands.
I've used finder, disk utility, and toast 6 and 7 to burn.
The maximum speed I've been offered by any of those applications is 2x.
It takes 30minutes+ to burn a full disk, so it's not burning part of the disk at higher speed, part at slower speed as has been suggested to me previously.
I've done a complete reinstall of the OS, no difference afterwards.
Problem was there in osx 10.3, still there in 10.4.
My local apple store did say they'd have another look after they replaced the drive, but I need the laptop for work every day, and can't really afford to be without it while it's checked out and repaired again at the moment. I'll have to wait I guess.