Okay, as promised, I got round to trial-fitting our Maxi Cosi ISOFIX base and seat this afternoon (sorry for the delay!).
To begin with, here's what the base looks like. Must add that it's surprisingly heavy - must be well reinforced. The front "leg" extends into the footwell to brace the whole unit:
And this is what the actual ISOFIX connectors look like ...
... and where it will be shortly fitted to (this is the nearside rear passenger seat of an A3 Sportback). The two open lugs at the base of the bench are the connectors and have a strip of metal across them:
You press the big button at the front of the base to extend the connectors out, then push the base home. Once the connectors slot in place (with a loud click), the indicator on the button goes green:
Then, you continue to push the base against the rear seat. There's more clicks as it settles against it - this stops the base moving forwards or backwards:
Note in the picture above that the base is pushing hard against the seat upright. You can now check that the front leg is also secure, by adjusting its height. If you got it right, the indicator at the bottom of the leg goes green, like this:
That's it! You now just drop the baby seat on to the base, and again, a little green indicator lets you know it's in place (these indicators are on both sides - nice touch):
And this is what the whole lot looks like:
We chose the Maxi Cosi product because of the fool-proof indicator system it comes with. It's not an electrical system, but mechanical so is perfectly reliable. Once in place, there is absolutely no forward or backward motion possible - it's just solid. However, there is some lateral movement, but it is still secure and wouldn't move under hard cornering.
I must just add (as per my previous post!) that I can't totally recommend this product yet, as our baby isn't due for another six weeks!