The Honeywell CM901/907 (wired) or CM921/927 (wireless) are very good. The 1 versions have a 24 hour timer, the 7 versions have a seven day timer. I've had a CM927 for over five years and it is very reliable. The optimization feature has always brought the house up to temperature when we wake up, no matter how cold, or warm, it has been overnight.
You can usually get brand new ones on Ebay for reasonable prices.
The TRV on the rad in the room where you fit the stat must be set to max or it could conflict with the new thermostat.
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