Just discovered that the devtoolset-2 ant is broken.
The issue is fairly simple: the files /opt/rh/devtoolset-2/root/usr/bin/ant
and /opt/rh/devtoolset-2/root/etc/ant.conf both refer to the wrong root file
system, and need to be amended in various places.  
I.e., / -> /opt/rh/devtoolset-2/root/

In the meantime I also tried devtoolset-3 (I guess still experimental?): rpm
-i
http://ftp.scientificlinux.org/linux/scientific/6.5/x86_64/external_products/softwarecollections/yum-conf-softwarecollections-1.0-1.el6.noarch.rpm
Installed devtoolset-3, and realised that the version of ant from that
install refers to maven30 in various places. Change that to devtoolset-3,
and it works.

Hope this helps others when they get stuck.

Niels