Stanleysteamer said:

Really, really pleased for you.
I am pleased that the climbing the hill in a high gear was useful.
So are you going to stick with the BDL needle?
I'm not sure what caused the crack-backfire. Maybe it is still a tad advanced. Perhaps there is wear in the timing chain or summat.
But still very relieved that at last you really are getting there!
I'll raise a glass or two to your success, this evening!

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Yes, I think it is still a tad advanced. I went out again on normal flattish roads and it kind of "bogged down" a bit and did the crack-back thru the intake. I turned it round and went home and pulled the plugs. 123 were rather "sooty" but 4 was much cleaner. I am going to retard the timing just a little more and have another go. I don't thinks it has had a long enough trip to really get the plugs hot enough on decent revs to get a true "mixture-picture" yet. But yes we are getting there, today's little run up the hill was very "sporty".

DanClarke said:

Yes, I think it is still a tad advanced. I went out again on normal flattish roads and it kind of "bogged down" a bit and did the crack-back thru the intake. I turned it round and went home and pulled the plugs. 123 were rather "sooty" but 4 was much cleaner. I am going to retard the timing just a little more and have another go. I don't thinks it has had a long enough trip to really get the plugs hot enough on decent revs to get a true "mixture-picture" yet. But yes we are getting there, today's little run up the hill was very "sporty".

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Well you've not said so, but I get the feeling the BDL needle seems to be the right one.
I guess it is possible that the timing chain has stretched a tad which could cause what you are getting. timing figures are just a starting point.
From now on you are feeling your way. But you are on the right track!
They go like sh!t off a shovel don't they?

After a bit of searching it seems to be either the timing being a tad advanced or the needle leaning it a tad too much.
But then you wouldn't abuse a car normally like that you'd use less throttle or change down. So only done for test purposes and you were looking for pinging not backfiring. So it does seem you have got it very nearly right.
Check to see the dashpot still has oil in it though.

Still very chuffed you have got it so much better!

I wonder how many youngsters would even recognise pinging if they heard it?

I once had to change a timing belt on a Gfriend's Cortina 2 litre at the side of the road.
Somehow I got it into my head that I would have to retime it, dizzy-wise, once done. (Thinking about it later I realised this would not have been necessary.) So I did it by looking for and listening for the spark as I twisted the dizzy with the ignition on. timing set up for static.
The car started and ran but pinged as soon as she gave it a bit of welly. Timing light, which i had left at home, sorted it out. She at least could hear pinging even though she didn't know what it was! She drove it for a few days until I got around to sorting it. Pulling the advance pipe off the dizzy would have stopped it temporarily.