Thanks for the tips! Got it sorted now... I was reading through your responses last night and just had to go out to the shed and work on it. In spite of having a cold and it was cold in the shed as well, I opted to nibble away at the broken stud, I was just too nervous to carry on with the stud extractor. It did take a while but the shed soon warmed up and had the radio on, so I was well entertained for the evening and it felt better than sitting in front of a computer screen.
The original hole was a bit off centre, but not so much that it was fouling the threads. I aimed to cut a slot so that I could try using a screw driver to budge the stud. The stud had sat in a good pool of WD40 for a few hours. I then nibbled away with a Dremel as Trevor suggested. Welding is not my thing although I wish it was, one day maybe? The hardest part was to determine just how close to get to the threads so that I could then just pull out the 'thread coil'... ha ha, this was not to be! It was a painstaking long effort with a needle and tweezers before the last crescent of metal was pulled out. I found it helped in having a straight cut bolt which help in lifting the shads out.
Having finally got the job done, it really felt satisfying to have successfully saved the original threads (Talk about rivet counting Ha! ) and they are in pretty good condition to be able to take a new bolt along with plenty of copper grease. I think the problem was that the stuck stud was just a tad tight in its fit, the screw in extractor must have pushed out the studs sides making it a tighter fit still.
All in all... a satisfying evening! Thanks for the tips...
P.S. Got the dating certificate this morning to say the B33 has matching numbers and was dispatched to Craze Brothers in Bournemouth 26th August 1953, this surprises me as the date on inside of the timing area states the engine was made on the 6/8/53, seems that BSA did not waste much time in shifting the newly built bikes into the show rooms.