East/West split is actually quite favourable these days, as PV payback is all about replacing self use now, rather than the FIT (which doesn't exist anymore).  I have the sunsynk 3.6kW unit - and it works great with batteries in our garage.

Trying to answer your specific Q:

1) Practically yes, even though I think the manual asks for a 40A breaker, no idea why.  Our 3.6kW sunsynk Ecco is on a 10mm^2 cable (for low Vdrop) and a 32A breaker.  When I first got it, I tried it briefly with a regular 13A mains fused connection, and it worked fine like that.  It's really not recommended though, the fuse will eventually fail, you need a breaker.  Nb:  generating equipment needs it's own breaker.

2) We use the 280L Seplos box (it's got wheels!), with 16 off 304Ah cells giving 15.5kWh, and it works great.  I don't see why you couldn't leave a few out, get a lower capacity, but why?  Ours charges up on "go" at 7.5p/kWh, lasts all winters day.

3) Yes, so long as you wire it as a conventional home battery, and not as "UPS".  UPS style is how it's used in South Africa/California, where the mains is dodgy and trips out a lot.  For UPS, the mains goes into it, then comes out of a separate cable and into your fusebox.  Conventional home batt there's just one mains cable.

4) Dunno if outlawed, but I think replacing a FIT inverter with a hybrid one is asking for trouble!  You're correct in that if you charged up the batts, then exported, by default they would pay you a wodge of cash... I doubt it would work in your favour for long...don't do that!

5) ?, though you can extend the twisted cable a long way so long as it's routed away from other current carrying cables

Great thanks a lot for the reply!

On point 4), I wonder how to wire it in.

I could leave the FiT setup as it is.

And then connect the Sun Synk separately and connect any new panels to that. But then i'd miss out on FiT for my new generation....

Can I route the battery discharge output separately? May be a question for sunsynk.

I can't be the first person to add a battery to an old FiT installation.

Part of the FIT agreement states that any changes to the system must be declared and approved. Since the FIT scheme stopped taking new applicants quite some time ago, I very much doubt that adding additional panels and battery storage will be acceptable.

Thanks for the reply!

So..... perhaps I should leave my FiT meter, inverter and panels untouched.

To begin with I can install the sunsynk inverter and battery without any panels.

Can the sun synk inverter behave like my Solic 200 and route any excess generation from FiT installation to the battery?

I can add new panels to the sun synk at a later date.

EDIT: I think this actually describes my situation:

So keep both inverters. Interesting.

@wasz 

Yes!  To be clear, we have 10 year old FIT PV, and more recently a 3.6kW sunsynk Ecco with a 15.5kWh battery.  The 3.6kW Ecco inverter charges from go electric, and it can also charge from the FIT PV.  The pic below shows a 24 hour period from the Sunsynk logging software.  The slow moving cyan line shows the battery SOC rising to 100% in the "go" time, then slowly falling most of the day, gobbled up by the heatpump, except when it's sunny and that line rises again 🙂  We have no PV directly connected to the Sunsynk (yet).

@wasz Note:  If you're adding a 3.6kW inverter to a 1kW system, you'll have to ask your DNO if that's ok.  G98 is the form I think.  You get a total of 3.6kW export for sure (you merely "inform" them of this), any more and you "ask".  You can do it yourself, it's usually free I think.

i have a similar set up albeit with a 5kw sunsynk, works like a dream. FiT scheme isnt compromised, just need the correct application/approval.

main thing is to make sure that nothing can be fed back to the grid from your additional inverter. just set up the sunsynk to harvest all the power from your PV system, to charge the battery ! 

you will need G99 approval .....!