Finally some progress!
I used my new tool to press the old journal bearings out. I was expecting this to be a complete arse, but it wasn't too bad!
Next, fit the new journal bearings into the other part of the tool, and tighten up the screw a touch (not as tight as possible, just 'wrist tight')
Heat up the casting (every little helps) and smother the surfaces with engine oil
I didn't take photos here, as I was terrified of it all going wrong! Place the ring containing the new bearings over the hole, aligning the split between the bearings with the marking on the casting. Now use the other part of the tool to press the bearings from the ring into the casting - this keeps everything straight and aligned.
Done!