The Honeywell in your photo is wired up in a manner that is normally associated with heating systems that don't allow independent control of the hot water and heating (ie those where the plumbing restrictions mean that it is not possible for the heating to be 'on' without the hot water also being 'on') Normally wiring like this is seen as a bit of a fudge and is usually associated with antiquated systems that have pumped central heating and hot water system that circulates by gravity. I've never seen this set up with Megaflo.
However, the Hive dual channel receiver doesn't have the heating and hot water common (COM) terminals available for use. They are permanently connected to 'live' inside the Hive, which is fine for fully pumped systems with motorised valves.
The wiring can be reconfigured to provide the same outputs from the Hive as are presently provided by the Honeywell.
From your photo, the Honeywell looks to be wired as below. I haven't shown the connections to N & L as they are the same for both. (With the exception of the link wire fitted to the Honeywell L)
Connecting & configuring the Hive as below will provide the same live outputs that you have now:
However, you also have a Megaflo, which are not suitable for gravity hot water circulation and they come with their own motorised valve, so I suspect the Honeywell is only part of the story and there is a bit more going on.
The above configuration will replace what you have now like for like, but will not give independent heating and hot water, so it would probably best to get an expert in to advise you on the best way forward.
