O.k. problem solved.
Sadly not a new species for Hong Kong, but it is still a first record of a female of the species in HK.
The species in question is Symmoracma minoralis (Snellen, 1880) - the male is much slimmer in wing and body, hence my failure to recognise the species straight away.
Still a good record of this uncommon species. Do you have the record data, please?