Our new tariff with eon cheaper rates at night. Found out our hallway radiator is a buffer radiator 

OK.

To minimise costs and maximise efficiency you are going to need to get your 'weather compensation' curve as low as possible so you are running with the flow temperature (the temperature of thee water produced by the heat pump) as low as it can be consistent with heating your house.  If you haven't already, I suggest you read this introduction to heat pumps which explains some of the features of heat pumps including this one.

In essence the process to optimise is this:

1. Set your heat pump to operate 24*7

2. Because you have a Halo (at least I think you do) disable the 'room influence' setting.  If you dont have a halo then turn any thermostat up to maximum

3. Turn all TRVs to maximum or remove the heads

This will result in a house that is too hot - so now:

4. Slowly adjust the weather compensation down until the house is just below the required temperature, then turn it up one notch.  By slowly I mean no more than one step at a time no more than twice a day, reducing to once per day as you get close.

5. If rooms are settling at different temperatures (or more precisely if the temperature differential between rooms is not what you want - eg you may want bedrooms cooler than living rooms), you will need to 'balance' the radiators using the lockshield valves (the valves opposite the TRVs).  We can provide separate instructions for this

The process takes a couple of weeks (which is why installers rarely complete it).

Once this is done, you have the system operating at its most efficient point and should provide a high degree of stability throughout the season.  If there are a couple of rooms that, for example, suffer from excess solar gain, you can reinstate TRVs in those rooms but by and large the majority of TRVs should be at maximum or the heads removed.

Ultimately its you choice if you wish to do this.  It may, depending on how you are currently set up, save in the region of 10-20% running costs and result in a higher level of comfort.  However it does require patience and a level of trust in the theory.  It also requires that you 'see it through', stopping halfway may make things worse.

If you do wish to do it please say so and I or another can talk ou through in a bit more detail.

I would worry about the heat loss from the cylinder and pipework in the loft. My cylinder and associated pipework have turned what was the coldest room in my house into the warmest room but at least that heat loss is in the house. 

Posted by: @johnr

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I would worry about the heat loss from the cylinder and pipework in the loft. My cylinder and associated pipework have turned what was the coldest room in my house into the warmest room but at least that heat loss is in the house. 

In fairness sometimes it's the only viable location.  So long as gge pipes and tank are well insulated it may be the best that  an be done.

My cylinder is in a small airing cupboard which, unlike with the previous cylinder, now barely gets warm.  We actually instructed the plumber to omit the pipe insulation in this cupboard so we would get some heat for airing!  If yours has as much effect as you say something is wrong.

@jamespa similar for us.

Our ashp cylinder is in the airing cupboard where very few pipes were insulated.

So after I insulated them the other half complained the airing cupboard was cold so I removed all insulation.

Now, at least the heat loss is still going into the house so what does it matter?

Posted by: @westkent

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Now, at least the heat loss is still going into the house so what does it matter?

Only to the extent that the heat has been produced at a lower COP than your space heating.  But as you are probably talking 1-2kWh max per day, who cares?