From the Friedland range that work with upto 16V~ power input, I chose the Freedland D117 Ding Dong chime by Honeywell (see photo). This can be powered by an external transformer with outputs between 8V and 16V, or by 4 "C" size batteries (so loads of free space in the case to fit a transformer). It has a modern, minimal and simple design, that would suit most homes, and the cover is flat, so maximises inside space. Funny enough, the inside of the chime looks identical to the one shown on the official Nest Hello installation video, so probably a good choice.

Note on the first photo that the transformer output cables run upwards one on each side of the transformer. I then soldered the short strip of mains blue wire to the neutral wire in the transformer and soldered the short strip of mains brown mains wire to the other input wire on the transformer, so wiring colour makes clear which is which.Then one day, I came home after a morning out, and returned to the Chime box making a horrible loud buzzing sound. I smelt burning, could see some smoke and when I touched the Chime box, it was VERY HOT and had melted somewhat. I wanted to give an update. I had been using my setup without issues for around 6-8 months. The Chime box when touched was always warm where the transformer was based and also where the Chime Connector was, but there were not any issues.

I am sorry to hear you had problems with your set up, and I fully endorse your recommendation for safety. One year after installation, my hardware is still going strong with no issues. I fully endorse your recommendation for safety. I then insulated the soldered joints with heat shrink tubing (you can also use insulating tape for this).

This transformer plastic casing is welded shut, so I decided to chop it open. I used the Dremmel tool with a slim cutting disk (second photo) and cut through the plastic weld on all 4 corners. I then slipped the medium size flat screwdriver in the gap I just cut and twisted it to split the case open (be gentle). I did this on all 4 sides. Cut the mains power, and replace your previous chime with this one. Connect it to mains using the other end of the cable connectors (brown wire to brown wire and blue wire to blue wire). A dremmel tool with a small cutting disk - ALTERNATIVE: you could also use a stanley knife(harder work) The only compatible chimes I could find in the UK are from the Friedland range by Honeywell, as they work in the 8V-16V AC range.

First thing to find was a wired door chime that works within the same voltage range as the Nest Hello and has enough space in the casing to fit the transformer and the Nest connector. BE CAREFUL TO NOT PULL ON THE WIRES AS YOU MAY DAMAGE THE TRANSFORMER, AND MARK OR WRITE DOWN WHAT WIRE IS CONNECTED TO WHAT BEFORE CUTTING THEM. The Nest customer support is near non-existent, or at least was until a couple of months ago, when I gave up trying to get some answers from them on Nest Hello issues on the Nest App for IOS. Some small wire connectors (see photos) - the ones I got came on a strip of 12. I only used 3, as I soldered together some of the wire joints (more compact than connectors)They are for illustration purposes only, as your home's push button wiring set up may be different from mine. If it works normally (ding dong), you are ready to install your Nest Hello. Follow the instructions on the Nest Hello app (not the ones in the video), selecting the version for a 2 wire connection. If you are connecting the chime on it's own (i.e. no Nest components) as per the wiring diagram on the prior step, and works ok when testing it, then install the Nest Hello following the instructions in the Nest app (the ones AFTER the video) for a 2 wire installation. All the best in however you decide to fit, but had to tell my story so you have all the details before making your own choice. For the transformer to fit in the chime case, I had to get rid of the plug-in plastic box it came in. This needs to be done with care, to avoid damaging the internal bits you will use.