I read an interesting method of getting ONE of the pins out Dave.. you drill a hole through the cover facing the pin and knock it out with a drift though the hole than weld it up again . A bit drastic I know but those things are absolute b*stards to remove. Also on the valve seats the guy TIG welded a bar across the seat and put a bar through the guide and heated the head and tapped it out.
Interesting methods Ash. Did you get your pins out without resorting to the drilling /welding method? I do wonder if Haynes actually removed the pins or just left them in and if the rockers are ok why would you bother to take them out anyway (and as long as you didn't vapour blast the rocker cover, that would be a mistake!). I certainly wouldn't want to do it again having had them snap in the casting.