Assigning an IP from the /29 to an interface means I can ping that IP from an external address. That shows, at least, the block is being routed to the main firewall. Couldn't get any internet access, 1:1 NAT etc working though. Have now run out of time trying to diagnose, but ISP has swapped my /29 for a /28, which is more than enough for what I need for now even considering the 4 lost addresses (3+1 interface) - so have just put it on an interface on the L2TP instance and gone the easy way.
Thanks so much for your help though - absolute legend!