I'm now trying to add a new LAN interface on Port4 which will be a seperate VLAN to ports 1 through 3 but the web interface just won't let me add the interface.

The steps I'm trying are (2960 is on 192.168.0.1/24):

LAN -> Switch
VLANID = 10 [ Member's LAN_Port_1, LAN_Port_2, LAN_Port_3] [ Untag LAN_Port_1, LAN_Port_2, LAN_Port_3]
VLANID = 20 [Member's LAN_Port_4] [Untag LAN_Port_4]

LAN -> General Setup -> Add
Profile: HVLAN
Description: BranchLAN
VLAN ID: 20
Priority: 0 (or 1, 2, 3...)
Mode: NAT
IP Address: 192.168.11.1
Subnet Mask: 255.255.255.0/24

Click Apply and nothing happens.

Either I'm missing something obvious or it's another firmware bug ?

No idea I'm more curious as to where you've got the beta software from. It's a rather large jump in number version.

I got it from UK support whilst trying to solve problems with VPNing

Strange the latest one they just offered me is version 1.0.7

they'll all say 1.0.7 pretty much, check the revision number, that's what changes and there are lots of revisions of 1.0.7 betas

But not version 3.0.7
Anyway I ditched the last beta from draytek as it didn't resolve upnp on the local network and broke load balancing.