@bridgetjohn Do you mean single- pipe like the link:

BUILDINGSERVICESINDEX.CO.UK "https://www.buildingservicesindex.co.uk/entry/136545/AEL-Heating-Solutions-Ltd/Single-or-two-pipe-central-heating-system/"

Like that?

@cycleneil Hi, yes, that's what I understand.  Ours is apparently single pipe and that is what is potentially causing the rushing water noise in the towel rail which links directly off the pipes in the airing cupboard before they disappear into the floorboards and service the rest of the radiators.

@bridgetjohn Oh dear. I’m surprised that the installer didn’t convert to a 2-pipe system as a routine measure, the 1-pipe systems date back to the early days of central heating and never really worked well even then.

@derek-m

Installer has proposed

"We have discussed  your system and feel it would help if we separated the heat pump from the existing system by reconfiguring the buffer vessel to a low loss header and installing a secondary pump to the heating system. This will mean we can control the pump speed around the existing system which should in turn reduce the noise issues. "

Any thoughts?

also see above about suspected Single Pipe system.  The house was built in the 70's and extended late 70's. 

Thanks in advance.

@bridgetjohn that is the solution I have - see Worcestershire ASHP. 

The purests would say this adds complexity and reduces efficiency. Which is probably true. Ideally you’d change radiators and reduce flow temperature. 
I’m getting SCOP of 3.3 controlled via a Nest. I will change to weather compensation from end of April (after 1year ownership) to compare and hopefully improve SCOP. And my bungalow is warm with no radiator changes 

Update on this. The bathroom towel rail plumbing was re-routed so that stopped that noise. The low loss header added and that reduced the flow noise in the airing cupboard. 
apparently if we want to add underfloor heating downstairs we’d have had to do that anyway?  
we’ve also added a bit of soundproof board to the adjacent bedroom wall but don’t know if that works yet as heating is off for summer.

just left with very large electricity bill for the two months when first installed with faulty PCB board so we had to use Booster all the time and run on leaving water temp thermostat.  Really bad that Daikin supplied faulty kit and didn’t compensate us for the consequences.  It’d be like buying a faulty car and getting a huge fuel bill because it was running inefficiency and then refusing to rectify it or compensate.  

I don't know if this helps, but you can change the pump limitation parameter to restrict the top end pump speed. It's in Installer Settings, so you'll need to change the user profile to Installer. From there, it's under Installer Settings > Field settings table > 9-0D. Change the value from whatever it is (probably 0) to 4. This will cause the pump speed to run at a maximum of 21lpm on start up. I found the default 30lpm much too loud and found no downsides to dropping this to 21lpm (which is 60%). Since my radiators were happily warm enough at the reduced 11lpm after initial 20 minutes or so, I was fairly sure it'd be fine at 21lpm as well. Sure enough, all rads warm up nicely and maintain their heat. Virtually silent running now.

It's all in the Installer Reference Guide.