Gerard,
Unfortunately you do not have D1.5. However, I think we are close to understanding eachother here. Your frase on the re-entrant angle of the toe of the weld is exactly what the issue is. As far as the code not specifying this, they do, by showing on the joint detail that the fillet weld is on top of the groove weld reinforcement. How much reinforcement is there is not an issue. It is then up to the designer to specify the fillet weld size in our case 1/4". which means you want the re-entrant angle of a 1/4" fillet weld. The fillet is not there for strength but purely for the re-entrant angle it gives. As long as the groove weld reinforcement has the same re-entrant angle as a 1/4" fillet you are good to go, or have a seperate 1/4" fillet weld to achieve this angle.