Thanks you so much tqhoang for your help, I followed your advice and successfully flashed the drive with 1.p1_rpc1 and it works perfectly now (I had used the SLAVE bootcode flasher to make the drive MASTER so I used the Slave main firmware as you suggested).
My 7550A is now recognised as Secondary MASTER and I no longer get the IDE #1 ERROR on boot and is also recognised correctly in Windows XP. I ran Nero's InfoTool and the drive checks out ok and is of course now RPC-1 too.
I did some test burns and they went fine, I also tested some bootable CDs and they booted up without any problems.
Thanks again tqhoang for all your help.