I solved my own problem! For the benefit of others who encounter this card, here's how -
1. Use "xf86config" to generate a new /etc/XF86Config file. Select "Cirrus GD543x" from
available drivers. (If not available, use "Cirrus GD542x" instead)
2. Edit /etc/XF86Config. Find the lines
# Device configured by xf86config:
Identifier "Cirrus Logic GD543x"
VendorName "<whatever you entered when running xf86config>"
BoardName "<as above>"
#VideoRam <bytes of video ram, e.g. 1024>
Below these lines, add
# Force detection of clgd5436 as earlier clgd5430 - (if version of Linux
# lacks 543x option, substitute highest available 542x driver name)
The next 2 lines may be present already, so just alter them, otherwise enter
them (for MemBase, enter frame buffer address that your card uses)
3. Save the file and run X - your system should now recognise the card as an
earlier version. (Note: I did try setting Chipset to "clgd5434", but this caused
display instability when dragging windows around the desktop)
Many thanks - I hope this may be of help to someone else.