As you have figured, you don't have the necessary neutral at the thermostats now. So, unless you wan't to run new wires out from the wiring centre to both thermostats, and have the receivers on the wall where the thermostats are now (not a good look) then the best way would be to mount the Hive receivers in the "airing cupboard" next to the wiring centre (check the location is as per the Hive instructions) and the Hive thermostats should then go where the existing thermostats are located.
I will explain how I would do one of the thermostats, using a single channel version of Hive. The other will be a repeal of the same process:
1) Trace the two wires from one of the thermostats back to the wiring centre, either physically or by using a multimeter. Note which two terminals the wires are connected to and remove the old thermostat cable from the wiring centre. [Hint: one of the wires will be connected to the brown wire of the motorised valve]
2) From the vacant terminals where the two thermostat wires have just been removed from, one should be a permanent live. Test with a multimeter to find out which one, and connect that to the Hive single channel terminal (1) Common
3) The second terminal will be for the switched live, connect that to the Hive single channel terminal (3) Heating on
4) The Hive needs a permanent live to provide it with operating power it, so this can be done by linking Hive terminal (1) Common, to the Hive (L) terminal
5) The Hive also needs a neutral to provide it with operating power it. This can be done by connecting the (N) terminal to the same connection as the blue neutral wires from the motorised valves.
Repeat the process for the second Thermostat, be careful not to install them the wrong way around so that the top floor thermostat is controlling the lower floor radiators and vice versa.
zone valves what are fitted in my airing cupboard.
As you have an "airing cupboard", that suggests that you may possibly have a hot water cylinder too? If so, the wiring is more complicated as one of the Hive's would need to be a dual channel version and whatever device is controlling the hot water will need to be removed and its wiring moved over to the dual channel Hive. If you don't have a hot water cylinder, then two single channel Hives will do the job as described above.