to add to difflocks info, presuming it's a new build and not connecting to original plumbing, it's not the hot water tank that has to be high as possible, it's the cold water header tank... which provide the pressure to the system for showers etc, so the hot water & cold water are the same pressure, your mains supply feeds this cold water tank via a ballcock. As in your standard "Airing cupboard" you have the hot water tank at floor level upstairs, a storage space above it with lat shelves up to ceiling height, and the cold water tank directly above in the loft. You need another (smaller) header tank (usually in the loft) for the boiler circuit - this is the "open vented" bit. This is also mains fed via a ballcock, so both tanks need overflows!!!
Having installed a similar system, I found it helped to draw it out in 3D with red and blue pens for pipe runs, I used 28mm for the whole boiler circuit, you also need a radiator in this circuit (usually in the bathroom) which has to be open "on" all the time i.e. must not be able to turn off, (this is to dump heat and prevent the system boiling) I came off the 22mm with 15mm to the rad, one for the feed and same for return.
The boiler circuit must also have a drain valve at the lowest point of the pipe run to enable you to drain the circuit should you need work on the pipework etc. (I put mine behind the boiler on the Hunter stove) I put stop valves in the CW feeds to the tanks, saves having to turn the water off to work on the system. There are additives you can get from a plumbers merchant, one to clean out the boiler circuit from flux etc during install and another to leave in the system to help prevent boiling/calcifying, don't add these until you've tested the circuit hot, two of my boiler connections had slight leaks and I had to drain the system to fix them (doh).
I could take pics of the pipe work if it'd help
edit: Think about forward planning with the HW tank, will you be wanting solar hot water in the future kind of thing, or HW from another logburner on the same HW circuit, if so it'd be worth getting a HW tank with 2 coils so it's an easy add on later rather then swap out the HW tank