After finally getting round to checking on the ground, Mallard Close (w892744465) is a footway corresponding to the position of USRN 20901846.

Of the two highways formerly mapped as Mallard Close, w29118416 is actually a gated service road for garages belonging to properties on Osbone Road and w29118389 appears to be part of Brinkworth Way.

Updated in https://www.openstreetmap.org/changeset/97061868