Ask him to note the use of the two port valves. They act as a high temperature non return valves. The call from the zone powers the secondary pump and activates the valve. The orange is switch live as normal and the boiler controls the primary pump. Piece of cake, but there's a lot of materials and work there, so sit down when he gives you the price.
I was hoping to use a single pump to drive all 3 zones on the secondary side (I actually have 3 spare pumps from previous efforts to resolve my issue, so cost of buying is not the issue, but my thinking is a single pump would keep running costs lower). The downstream circuits all have a relatively low head loss (< 2.5m) compared to the boiler which has a high head loss across the HE on the primary side (for which I have my Grundfos 25-80 if needed). The only issue I see here is that all 3 zones would be teed off of a single output from the LLH, rather than having individual outputs from the LLH (as per your picture). Any issue with that?
One other question please - how does a LLH behave when one zone requires a much higher temperature than the others? If the rads are heating slowly from cold and someone jumps in the shower whilst the HW is on, the cylinder will need reheating, but at a relatively higher temperature compared to the rads. Can the LLH help with this scenario by allowing the higher flow temp to the cylinder whilst a lower flow temp feeds the heating zones? And if so - would this necessitate a separate feed for HW from the LLH (or does it not matter)?