Thanks!
Yeah, I couldn't remember it feeling overly weakened when I cold set it after doing the modification and just to be sure I gave it a little squeeze and pull just by hand this morning and I honestly cant say that the deflection was anything out of the ordinary.
I might braze in some extra tubes anyway, I remember the loaner track bike I got at Meadowbank years ago was a lovely steel thing with braces between the chainstays and seattube and I've been looking for an excuse to replicate that.
Or I could add a whole mixte type tube as I did on my Flying Scot.