2.7 TDV6 Compression Adaptor

Hi everyone - just a quick one, I need to check the compression on my 2006 RR Sport 2.7 TDV6
The problem is my Compression Testor adptor ( all ) dont fit - all the glow plugs are out, does anyone know the correct size adaptor I need to screw in the glow plug hole?

Many thanks in advance -

From memory it's an M8 X 1mm thread.