OK, I used the Add/Remove Software tool to search for radeonhd and found one entry. But this text in the description window raises some concern:
X.org X11 radeonhd driver for AMD GPG r5xx/r6xx Chipsets.
This package is a snapshot of a work in progress. You may experience regressions, bugs, errors, broken displays, and other undesirable phenomena.
I have no problems with vesa but the whole point of Linux for me is to experiment and this one didn't work out.
Thanks for your help anyway.