It's Richard Bros that are asking me to confirm the lengths, as I am trying to buy spares. I think they are using this as a check that I am asking for (and they are about to send) the right ones!
(Seem to be about 7" and 4" but hard to measure accurately when mounted in a wheel.)
Having passed the 7" and 4.5" spoke lengths to Peter Richards in Cardiff he is querying the lengths and asking me if I am sure that they are right.
Looking at the diagram on page 525 of the Source Book, the dimension shown is the straight bit of the wire before the kink - so perhaps the overall are dimensions are greater than these in practice - and as per his photo above?
The official Austin Lists of Spare Parts (see Assoc. archive) illustrates the various spoke lengths and the point they are measured from -I certainly wouldn't rely on the Source Book as gospel for anything.