Well once again thank you for all your help/information, I must admit that now I'm rather concerned about fitting the Helicoil insert, but Muskrats method appeals to me. I'm not in any hurry to fiddle about with my pushrod positioning tool etc, having taken the rocker box off and then putting it back on.
Rex, your mention of the use of 1/4"BSF makes sense, but whatever the thread was it just didn't go on beyond the first pickup, not even close. When mentioned to Draganfly, who did buy up a lot of stock from "the stainless steel man" who used to go to various auto jumbles back in the nineties, they offered to refund the cost of them if they were sent back. That unfortunately would've cost more in postage from Spain, so I didn't bother, just threw them in a box with other badly fitting so called BS cycle threaded products (26TPI), mostly in S.S. some from the chap mentioned earlier.