My sierra diff seems to leak oil out of the centre of the prop shaft coupling - maybe a teaspoon full per day. Is this normal?

Nope

Ah. So, is it serious? If I bolt my prop shaft to it and the gap fills with oil; assuming the prop shaft to diff doesn't leak, is there a problem? On the other hand, is it easily (read cheaply) fixable?

you can get a full set of sierra diff seals on ebay (two drive seals and a pinion seal) for a tenner

mine leaked through the centre nut hole. If you take it off and squirt some silicon around this may stop it.

when mine leaked from the front (not sure exactly where from) a new seal was cheap from ford, local garage fitted it free.

Would have thought that if pinion oil seal is leaking oil would come out between seal and pinion flange? . Sounds like the oil is finging it's way along splines inside pinion flange . May be worth fitting new seal and some sealant along splines? H.T.H. Rusty

just looked at it again. The oil is coming out between the steed flange and the brass coloured centre.

How do I get to the seal? Do I undo the nut, pull off the flange and it is there or is there much more to it than that?

its in the haynes manual if that helps, but i believe you have the gist of it there.