I'm guessing the Stuart pump shown is a loop pump for a plate heat exchanger (PHE) ?
I know some of the regulars here will have apoplectic fits just at the mention of thermal store*, but I'd suggest you would be better running the heating off the store and use the boiler only to recharge the store. Boiler flow goes into top of store, boiler return comes from bottom of store, and you'll need to somehow control the flow to get a higher than normal delta-T across the boiler so it can reheat top-down. Depending on the boiler, this may require a TMV. Ideally you want to put only fully hot flow from the boiler into the top of the store and avoid mixing part warm water with what should still be hot at the top.
* Just do a search, and you'll find certain regulars have a pathological hatred, and a completely closed believe that there is no situation whatsoever when a thermal store can or should be used. I'll just disagree and leave it at that !
Setup right, you should be able to have the boiler condensing pretty well all the time - which will most definitely not be the case when running the CH with a bypass loop.
Then take off a flow from around the middle of the store, use a fully modulating pump (eg Grundfoss Alpha II), and fit TRVs on all your rads (with no room stat) and do not have a bypass anywhere on the system. The CH return goes into the bottom of the store.
If the cylinder is an indirect cylinder, you have the option of using the coil as a heat takeoff for the CH, which you can either run as a separate OV setup, or as a sealed system according to preference. That should deal with the arguments about the rust and sludge from the radiators settling in the cylinder.
This decouples the CH from the boiler - the boiler will run at high power for short burns to top up the store. The modulating pump will match pressure to flow so the TRVs can do the job almost silently - and you get fully independent control of individual room temperatures. The key thing here is that you are not compromising the boiler by making it cope with variable demand from the rads, or compromising the CH by making it deal with the requirements (minimum flow rate) of the boiler. You just need a timeswitch to turn the CH pump on and off (independently of everything else), the boiler runs only off the cylinder stat (or preferably two linked) and there are no motorised valves required at all.
