Hi,
Tagging the VLAN from the controller to the APs was the missing part. Using HP Airwave, I had convinced myself that VLAN 240 was on hardly on any switches, this wasn't entirely true - it was on some of them, which was in turn the exact areas that DSHS Visitor was in use. To further muddy things, one area had DSHS Visitor but it didn't have VLAN 240 set as it's egress VLAN, so worked but not in the correct way by giving out addresses in another VLAN.
I've added VLAN 250 to key switches and ports, so now IPs are being given out correctly as they should.
Huge thanks for the help received here.