They aren't taking orders until May, with a view to start doing installations in July.
She said you can sell back excess (they call it "spillage") to the National Grid, but it may not be worth the capital costs (something like £150 to get an appropriate meter). So they don't recommend doing that.
She said a bill is going thru Parliament, they are hoping it will be done by end April, that will stipulate that no planning permission is required.
Although based in Glasgow they can install anywhere in UK.
The site says something like 0.5-1 MWh per year as what's generated. I reckon our daily usage is about 8 kWh, or 2.9 MWh (bill from Good Energy puts it at 2.5 MWh/annum). Sounds like it's worth it.
The turbines must be installed by them, and come with a two-year warranty (presumably on parts and labour), and any fiddling within that period would render the warranty void. What happens with maintenance after that I don't know....