Guys, Thanks for your comments.
Bob, sadly I didn't have the tools to get enough heat in the metal. Drilling was OK, but the challenge was keeping the drill straight as it seemed the casting was far easier to drill into than the old bolts
Of course the tap broke in the last hole as I was rushing to get the job done, before it got too dark!
Russ, the Walton tap extractors are also available for 3 flute taps....and even for 2 flute taps! Maybe you have to convince the guys at work to buy matching taps.....or just don't break them
Alex