Installing a Hive Thermostat

I purchased a HIVE thermostat and want some assurance before I go about installing this.

First of all i was wondering where to put the receiver as I thought it would just replace one of the thermostats however my thermostats only have 2 cables plugged into them so maybe needs to go somewhere else. Anyway I took my thermostat of the wall and the current cables show the following:

• A: Yellow / Green cable attached to nothing.
• B: Smaller Grey cable attached to nothing.
• C: Brown cable plugged into 2 COM on thermostat
• D: Grey cable plugged into 3N/0

The Hive reciever has the following:

• N: Permanent Neutral
• L: Permanent Live
• 1: Hot Water off (NC)
• 2: Heating off (NC)
• 3: Hot water on (NO)
• 4: Heating on (NO)

I have a combi boiler so dont really need to do the hot water part but am wondering if i can just put the reciever where the thermostat is for the zone i want to control or does it need to go near the boiler and then i connect directly from there.

Also if it does go in place of the old thermostat how do the cables go e.g. does the A -> N, B -> L, C -> 2, D -> 4? Or do I need to do it from the boiler / get someone in to do this?