Don't know much about boilers/heating but as this is a techy forum just pointing out that you can now get wireless setups with internet connectivity so you can monitor and control heating / hot water from computer or smartphone.
I see you said simple to use, maybe its for an older relative, the internet connected ones will be more complex but it would allow you to monitor their house temp and make sure they are not freezing themselves to save money or something.
Where have you read they aren't reliable?
There is nothing wrong with the DT20RF, I've sold plenty over the years and have had few back.
I've not sold any FR10 units or that combo , people just tend to go with the DT10, DT20 ( RF ) versions.
That FR10 looks like its a hard wired unit so would mean running a cable from boiler to where the FR10 unit is going to be located. You happy with that?
The unit that Hyburnate linked is also a decent, I use one at home.
http://www.honeywelluk.com/products...tats/TPI/CM907-7-Day-Programmable-Thermostat/
I have one of these works great on my Worcester boiler.
Need to check how many cores wired into your current stat and see what the FR10 uses.