The "anything you fail to say now but which you later rely on..." is peculiar to the version of the Miranda Rights used in the UK. We call them Miranda Rights these days, but it's an American term. It's worth noting that the European Court won't uphold that presumption of guilt based on silence or refusal to comment, and ruled against the UK government on the matter in 1996.
The right to remain silent existed in the UK from the 13th century until recently. You still have the right, but if you're being tried in the UK, your silence can be treated as if alluding to guilt. Atrocious really, how much of a beating our rights have taken in the last decade alone.
Fortunately for Symonds, this is going to play out in Europe, and away from this feckered up legal system.