Can't find a spline or spanner

I have a 2003 passat 1.8T, It is over heating, I can't find a spline to open the head cylinder, I tried size T50 it abit loose and T55 is too big, where can I get the correct spline or spanner so I can out in a new head gasket, please help

as I went to the booysen VW and they said they can't help