Cisco Switching/Routing :: DHCP Propagation In 3750x

I'm trying to get clients to pick up a DHCP address from my server. Physical config is as follows:

4-switch stack of 3750X-48P-S (x 2) and 3750X-24S-S (x 2). 48's are physical switch 1 & 2. Data stack is working fine. That's the Core Switch
Mix of 2960S-48TS-L and -24TS-L, 2960CG-8TC-L and 3560CG-8PC-S Access switches.

physical switch 1 in Core Switch, all ports are VLAN 4. No DHCP required - VLAN 4 is infrastructure only. All switches have:

Switch#(config)int vlan 4
Switch#(config-if)ip address 10.0.4.x 255.255.252.0 (Core switch is 10.0.4.10)
Switch#ip default-gateway 10.0.4.10

Physical switch 2 in Core Switch, all ports are VLAN 8 (client access, main corporate office site)
Physical switch 3&4 in Core Switch, all ports (SFP) are trunk

[Code] .....

So in that slightly spooky, obscure way that they have, the clients ought to be picking up a DHCP address from the relevant VLAN, depending on which VLAN the access port they're connected to is in. But they don't pick up anything. Connectivity is there - everything can ping everything, including clients if I set static IPs, and including DHCP server.

Should ip helper-addresses be set on the access switch VLAN interfaces too? Or have I missed something else obvious/critical? I need to have this out and on the ground and tested within 3 weeks - this is the first hurdle and I've fallen over. Like I said at the beginning, this config (or the equivalent) works fine on Packet Tracer for distributing IP addresses.