I followed the recipe exactly, but as I didn't need any extra milk for the dough, I brushed the biscuits with water before sprinkling on the final dusting of caster sugar. Saved me opening a carton of milk just to use a splash of it.
Overall the recipe, and the results, were disappointing. I found it really difficult to handle the dough once it had been rolled to a rectangle of the correct size - it was almost paper thin at that point. Once the filling ingredients had been added, it was impossible to roll out the rectangle to the correct size a second time, because the amount of the filling just wouldn't allow the dough to be rolled out far enough. I managed to get it large enough to cut out 12 biscuits, but they didn't look anywhere near as neat as in the photo accompanying the recipe, as I couldn't cut through the filling layer cleanly.
After this disappointment I checked other recipes online. The ever reliable Delia Smith uses a very similar dough - a bit more sugar and more milk instead of egg - but less than half the filling. I think this would enable all the currants/filling ingredients to become properly embedded in the dough.those published in The Guardian newspaper, so I can see why he reduced the sugar in the dough, added the egg and increased the amount of filling, but for me the change in proportions between the filling and the biscuit dough spoiled the recipe instead of enhancing it.
I won't be trying this recipe again, but now that I've tried barberries for the first time, I'll be using them again in my baking. I think they'd make a lovely addition to Christmas mincemeat.