Y Plan Sundial - 150V on orange wire, boiler stays on in heating only mode
According to V4073A FAQ:


  When this Heating demand is satisfied, during a Heating ON period, 50V to 150V (voltage can vary
  according to the supply) will remain on the orange wire...


So the voltage you've found is there by design. Unfortunately, it's enough voltage to trigger the boiler's relay when it's supposed to be off.

(As the programmer is in HEATING only mode, the grey wire will be live which keep the central heating/A port open.)

You have two options:


Use a relay - Wire in a strict 230V relay (Such as http://uk.rs-online.com/web/p/products/4334076/) between the boiler switched live and pin 8 on the wiring centre
Use the official "RC" device - The RC device (Worcester part 87161092750) pulls the voltage down below the tolerance of the boiler's relay. Only when a full 230V is present will the boiler fire. The RC device is wired between Neutral and the L/R (switched live) terminal. There's no polarity, so use either wire for each. In a wiring centre, it would be installed into terminals 2 + 8


I chose the RC device so that it was supportable and maintainable by others in the future.

Edit:

The official RC (resistor–capacitor) device is actually a Roxburgh XEB 1201 noise suppressor. I guess the resistance of this (120Ω) causes the voltage on pin 8 to now read 21V.
It comes in a Farnell bag, so pretty confident it's resold without modification. Buying direct would have saved me £20!