Button press not initiating ECG reading

You can try following the steps outlined in this link https://support.withings.com/hc/en-us/articles/360023653753. This explains what you need to do if you're having a problem taking ECG measurements. I hope it helps.

Thanks, already read through though and not anything related to my particular issue

Sounds to me like the button is no longer making contact.  If you have the right tools to press the back on again, you could take the back off, remove the battery, and then try to clean the contact on the button.  If not, you might ask a watch repairer to try this.  I wouldn't recommend a small booth type, find an old fashioned watch repair shop with a reputation to lose.