Well, I got it sorted yesterday. There IS metal in the frame, there’s a square aluminium extrusion running down the centre of the frame, but it was easy enough to drill through anyway. After MUCH faffing I managed to get the wire running down INSIDE the UPVC frame, but it took some effort – it kep getting caught on something inside the frame, possibly screws coming in from the other side relating to the quarterlight window? I ended up having to put a rod UP from the bottom and use that to hook the wire and draw it down. It now enters right behind the ring doorbell, and exits right down at the bottom. Means it’s nice and neat at eye level in the house, and invisible outside.
The electric chime now rings immediately when the button is pressed, and while the ring app does take a little time to respond, it’s noticeably quicker than it was before the power supply was added. The ring doorbell can be set up for old style metal chimes or an electronic one with various tunes available
Someone mentioned power – chimes are either 230v with a built in transforer, or expect an external 8v transformer to be fitted. I went for the former. Either way they should provide the same power supply to the push