I can recommend the "Station Master" app as it has 3D maps of all the tunnels and bridges connecting platforms at every station. You'll see clearly how the platforms at Euston line up. On the app webpage there are some example maps, including Euston: https://www.stationmasterapp.com/3dmaps.html.
It also has information on carriage and door positions. I don't think they're all up to date with the stock changing on some lines in the last few years, but it's still a great help. Only available for iPhone unfortnately. They sold the platform maps to an android app, but they're really poor quality and the app crashes a lot. I ended up buying a refurbished iPhone for £100 so that I could get it.
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