@quicksilver Thanks, yeah, I tried all the joysticktype settings including 2axis, which is supposed to allow two controllers. Didn't help.
However, this morning I tried renaming the default dosbox-SVN.conf in retropie/configs/pc. I then ran DOSBox again from +Start DOCBox.sh and the screen looked different and the keyboard started generating invalid characters. I quit out and DOSBOX had recreated a new dosbox-SVN.conf. I renamed that one and reinstated the original one then ran it again. All of a sudden the Thrustmaster is recognised ! No idea what has changed. I guess forcing it to recreate a dosbox-SVN.conf must have also caused it to update some other setting file.
Anyway, I'm happy it's working now. Thanks quicksilver for your suggestions.