New Thermostat

[mightyapu]

Hello

I have an existing aged thermostat on our first floor landing that i'd like to replace. It's a Honeywell, but i'm afraid I can't find the model number. It's served by 3 wires, one red, one blue and a yellow one.

I'd like to know which thermostats would be suitable to replace it with (probably from screwfix) and whether it's a fairly straight forward process.

Thanks for your time.