I am really impressed that you have tackled this job, certainly you have the satisfaction of having done the job, and also saving 6 hours of labour, which could be £300
I assume that a repair to the heater has probably been done before by someone in a hurry, and I wouldn't be surprised if when you look at the old matrix that it might have had a bodged repair done to it rather than a new replacement.
Have a look for a date code on the old matrix.
Well done best regards,
Rustic.
PS: Might get my wife's hair dresser to warm my hose.