High data usage in iOS17 for Push Notifications when mobile data for all apps turned off and connected to Wifi?

I noticed after updating to IOS 17, my cell data started running out real fast (Iphone 14 Pro), even when totally connected to Wifi (I work from home - so normally connected to my Wifi). Upon inspection, it appeared to be System Services and Push Notifications. I spoke to Apple support and they suggested a full factory reset, removing my VPN and also contacting my mobile service provider (Sky in the UK). Sky confirmed that I am using the mobile data at a super fast rate compared to normal.

I did all this and also turned off mobile data for all my apps, turned off push notifications for all my apps and monitored for 24 hours while at home. I reset the stats and in a 24 hour period when permanently connected to my home Wifi, my Push Notifications eat through 2.5Gb (yes Gb) of mobile data.

Apple support suggested putting on low data mode and went through all my settings. Another day later and its a bit better, but still nearly 1Gb of Push Notification Mobile data burned through when no apps have mobile data turned on. I only have a 8Gb / month cell plan and this is normally more than I need (for the last 15 months I accumulated 30Gb of excess data on this plan).

If anyone has any ideas, it would be appreciated as my phone is now effectively useless when not connected to Wifi as I have the mobile data turned off for every app and still getting through a run rate of 30Gb a month on push notifications.

I have another call with Apple Support tomorrow.