The radiator valves wont turn the Gas on and off, all the radiator thermostats will do , is reduce the temperature coming out of them. For example - if you had them, you could turn them down to 2-3, then that would keep the heat low in the house and keep the gas at a constant level, which will save the spike in energy when you turn it from direct cold to hot ! It does not do exactly what you want- but it is the nearest you can get to without having to spend money. Not having the numbers from 0-5 means you dont have the latest gadgets on the rads, but turning the + or - valve does the same thing, except you have to guess how open and closed your rad is. You can only know this by playing around with it.....
My suggestion before you spend any money - is to prove to yourself how much of a gain you would be getting trying all your options.
1st - for one day- check the meter number and see what it is currently set at ( document it )and use the gas by turning it on and off as the place gets cold. Recheck the meter at the end of your experiment.
2nd - on another day- check the meter number- turn the valves towards the - position, but untill your satisifed with the heat coming out of them and leave them like that for a 24 hour period. Again check and see what the meter number is
Check and see from your analysis which one uses the most gas and from that decide if you want to go with the alternate option of installing a wall thermostat and a timer....
I dont know if the variation usage is what you are trying to reduce , or if it is overall gas usage. I know Bord Gais charge companies a specific amount dependant on variation alonside usage- but I dont know if it takes much of a hit to a home owner- I know that for our company we have an SPC Hit and a variation hit ontop of the normal bill aswel as other stuff.
Check and see what your ROI will be on such and item and see if it ways up your spending. Heating your home while no-one is there is wasted heat, but I am no - one to argue if it reduces your bills.
For simplicty in what you are trying to do you need to put a thermostat on the wall and a timer with it.
The worst thing about the gas in my place is that when I turn my gas on for heating it heats the rads but does not heat the water, which seems ridiculous to me, but the builder said it was the way it was designed. I know I need to get someone in to take a look at it - as it is wasted heat and money in my eyes. I have to heat the water separatley through the gas via another button.
Hope this helps.