purpose of matching. Sources without a match in FIRST or GMRT but with a match to OMK2009 were classified as extended if $C_{\rm OMK2009} > 1.2$ and compact otherwise. All sources with a match in WSRT and not in the other three catalogues used here were classified as extended for the purpose of matching as the resolution of the WSRT map is not high enough to ensure these sources are not extended.

There are six sources which are not classified as extended or compact because they do not have a counterpart in any of the four catalogues used when classifying the sources but which do have a match in OM2008 or BI2006, and therefore an accurate position. These sources all have angular sizes less than 3 arcsec in the OM2008 or BI2006 catalogues so were considered to be compact for this purpose.

There are therefore 38 sources which are compact and 57 sources which have been classified as extended for the purpose of matching but which may in fact not be significantly extended on these angular scales. These two groups of sources are treated separately when identifying optical matches.

Matching the catalogues {#section:matching-details}
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![The separation distribution when the radio 