Solar thermal - heatbank question

Designing a heating system for my house and have a few questions someone might be able to help with
Am probably DIYing it and am pretty sure i'm having a 500ltr vented heatbank with external flate plate heat exchanger for DHW. Was planning to have dirty water in tank heated by back boiler on the fire in winter and 30/40 vac tubes on south facing shed roof, and put either 4 or 5kw of PV on an east facing roof which could be used as an immersion input for the tank also.

1)Can i run the 'dirty' water from heat bank through the solar thermal? It would seem a simple solution and more efficient way of transferring energy than a heatbank with a solar coil, and cheaper.. so can you not just add anti freeze to the dirty water? Will sediment accumulate in the solar tubes? Aren't some solar tubes either just metal fins or sealed units with antifreeze in and are more suited to this idea?

2) My PV idea is integrated and isnt a big difference from buying indian stone to replace the current york stone slates, when you take the overlap and wastage into account on a 20/25/30m2 extension on the back of the house. I want to just fill the space but will there be any extra costs from fitting a bigger than 4kw system.. i wont be bothering with the FiT. Do i have to contact my dno? Being east facing its only likely to produce 3.5 - 4kw anyway.

3) Lastly i have changed my mind were i would like to fit the 500 ltr tank and now want it on my first floor (70kg tank weighs) and have already got beefed up 9 x 4 by 4.5 metre joists in (Architect spect 8x3x4.5) one end on the wall the other 2 x 2bar reinforced concrete lintels at the other.. if i filled in with 4 9 x 4's will that take the weight of the tank or should i put a 8 x 4 rsj under? Will ask my architect to ask a structural engineer but he's always busy, just wanted an idea.
Many thanks.

Thanks, worth putting on here, just having a think about it this morning as better get it sorted.

Thanks Dave interesting stuff, just brushed off my pointing on the chimneys this morning, you think cr*p! scaffold will be round the back monday and so joists/rsj in ceiling will need to be right for store before i can get last 6 foot of skin up and roof on, solar etc... nowt like leaving it to the last minute

My thoughts on this are from the perspective that everything is new so i may as well have a simple solution, having a heat bank would save the cost of putting a water tank in the loft (would have to reinforce ceiling there=cost) so just use mains water pressure and a flat plate heat exchanger. Heat banks are cheapest without a coil in so wondered why you couldnt circulate just the one fluid through the radiaters and solar coil manifold? Is this because of rust or sludge build up in the system? It seems an elegant idea as the will be the most efficient heat transfer as there will be no heat exchanger. If not then a single internal coil for solar is probably what i will do and everything thing else on a sealed primary water system.

I'll have a look at the velux kit, its the French gse kit i was looking at and can be found for about £35 a tray so not too expensive (though expensive enough for a bit of recycled plastic)

I think you're looking at it the opposite way to me, i meant have primary water in the tank which never goes anywhere but round the system, radiaters, to and from stove boiler, under floor etc this seems to prevent the problems of furring up of coil internal heat exchangers as the primary water will never change and so minerals (in a hard water area here in the pennines) wont furr up the inside of the coils like you'd find in an old cylinder, an external flat plate would be connected with valves either side and could simply be taken off and back flushed with acid (what was it we used for the parlour one? Phosphoric acid) or taken apart every 5 years etc.

Not moving tank there is no tank so brand new and room it was in is now knocked down, just trying to find the best place for it and space wise first floor in new extension seems best.
Thought about the dump for uncontrollable heat source like solar (practically we are not going to pile the fire full of wood an bugger off though i no all situations have to be accounted for) we should be able to just use the radiaters in the heating system as a dump could we? I did have a few thoughts... have a big greenhouse to go in the back garden next to the shed i'm putting solar on and thought maybe heat store under that? Maybe a bit wild thinking

Must go be on till dark with my lambs if i dont get going. I'll have a word later.