L2TP VPN doesn't work over EE on IOS 12

We have number of devices which have an L2TP VPN connection to a Windows server.

It doesn't work for iPhone 8 devices running IOS 12 and using the EE mobile network - it fails to connect with "The L2TP-VPN server was unreachable. Verify the server address and try reconnecting. If the problem continues, contact your Administrator."
It does however work for anything else - for example:

• iPhone 8 running IOS 11 on EE
• iPhone 8 running IOS 12 on other mobile networks (O2, Vodafone, etc.) or Wi-Fi
• iPad running IOS 12 on Wi-Fi
• Android on any mobile network or Wi-Fi
• Windows 10 on any network

So, you'd think EE would be able to help?  No, as the problem only started with IOS 12 they say it must be something in the update - and therefore Apple's responsibility.  Apple of course say that the problem must be at the EE end, as it works with other providers.  So, short of downgrading to IOS 11 we're stuck.

I found this, but no solution is posted:
https://community.ee.co.uk/t5/Apple/L2TP-VPN-connectivity/m-p/737163/highlight/false#M87064

We suspect it may be related to EE's implementation of IPv6, but don't have any specific information.

Has anyone else come across this?  If so, how did you work around or resolve it?