there are two sump gaskets jon, a two piece which is early one, and a one piece,

is this a series 1 bottom end and sump u have put in? if u have fitted a series 2 sump gasket on a series 1 sump, it will leak where u have said

if u fit a proper series 1 sump gasket and yours is a series 1 bottom end, u will need a bit of sealer on the sides where the circle cut outs are