@Vornne (and by implication @Kayden):
Thanks, your suggestion worked perfectly! I had resorted to running the client under Windows XP, but now it's working on both (actually I think the graphics is a bit smoother in linux, but there's maybe a bit more lag). Great to actually be able to play worthwhile game on Linux though!!
@Barius:
I don't have an answer, but I did see in the data/config/gldrivers.xml file there was another rule which might need to be changed -
<rule description="Intel DRI: Disable texture compression">
try also changing that pattern to
<regexp string="glversion" pattern="Mesa 7\.0\.." />