Drilled out the as much wood as possible using a portable drill press angled at about 4 degrees (I decided on 4 degrees as I can shim to make a larger neck angle if necessary) with Forstner drill bits. After removing most of the wood I went "old school" with a chisel and mallet! All went OK. Lots of sanding and the neck now fits.
I think the fretboard is still a bit too high! If I lower the neck further, I will probably need a greater angle!