my ports \distfiles directory had grown to over 30Gb in the 4 years since I last did a complete clean fresh build and as its something of a test rig for trying stuff out, it had grown rather large.
by running the following commands I regained c22Gb of disc space, ready for which ever pet project I choose to tinker with next.
sudo portmaster --check-depends
sudo portmaster --check-port-dbdir
sudo portmaster -s
sudo portmaster -y --clean-distfiles