The lathes.co.uk site has some photos, and looking at these the swivelling head looks pretty simple, being an angle plate that clamps to the table with a single stud, with a block on the vertical face clamped by another stud. These two clamping axes hold the angles. The block then is bored for the mandrel that holds the tool. This could be your ER25 spindle or something of the same diameter bored MT2 to take Myford collets, with an appropriate thread if needed. ER25 is probably best as you have it and could hold drills too for 4 facet grinding. It's not really necessary to have engraved angle scales, though nice, as you can use protractors or make angle setting gauges to set the thing up.

You then need some way to index the spindle, and quite a good approach is used on the Quorn that has a plate with holes for division that clamps on to the mandrel using a clever collet arrangement. I made a version of this when I adapted my Quorn to take R8 collets which needed a new larger work head. I could probably supply some drawings of the Quorn arrangement which if memory serves uses a 1 inch mandrel which must be close to your ER25 spindle diameter. You also need an indexing pin on the block to engage the division holes. If it's of interest I could post some photos.

John.

Blackgates still sell the drawings. Mine came with the basic swivel tool head, and along with the drawings, i was not impressed with the basic design, so i made my own, with an extra swivel axis, graduated all round, hole down the middle is 1" parallel, large enough for my one -off collets, 3C collet holders and one off holders.