This is a great resource, thank you to those who have contributed. I've read through and seen this question come up a few times, but as I am looking to book my first 2-4-1 soon, I'm just interested to know a bit more about the practicalities.
It seems to the best way to "guarantee" the dates you want is to book at T-355 by booking the outbound online at midnight when the flight becomes available for booking and there should be at least two reward seats available.
The part I'm less clear about is getting the return flight. It sounds like this can be done over the phone at T-355 (either from same airport or open jaw) but have you not essentially used up your voucher in booking the initial outbound? How do they know there is still a 2-4-1 element left to be used up? It feels like if I were to do this, the agent would say "Yes sir, that will be X thousand pounds sir".
Presumably if you just state you booked a 2-4-1 booking online and would now like to add the return, they would immediately know what you're talking about and there would be no fee or charge to do this other than the taxes etc for the two tickets?
I'm a bit cynical but it all seems a bit easy to get exactly what you want when everything I've read up until now says you need to be very flexible!