Just updated my two other t-beams.
First one I used esp-connect and just wrote the mb203 to 0x10000. Worked first time.
Second one I used flash download tool. I changed the SPI speed to 80MHz. Used the bootloader and partition files from 1.8, just changed the app0 to use mb203. Reselected all the files so they are all highlighted green. Worked first time.
I have no idea why this wasn't working, but now it does.
One interesting thing though. This is the partition map from esp-connect:
Notice the bootloader starts at 0x0 and the OTA data is at 0xE000. This doesn't match what the instructions for 1.8 say.
The 1.8 partition layout is:
boot_app0.bin (8KB) starting at 0xE000
bootloader_dio_80m.bin (17KB) starting at 0x1000
SoftRF.ino.partitions.bin (3KB) starting at 0x8000
SoftRF.ino.bin (1691KB) starting at 0x10000
So this doesn't make sense to me.
I'm still confused, but all my t-beams are working now.