Ok Folks, it appears this problem/fault has now been resolved. It has now been 8 days, last Friday, since I heard from Mercury asking various questions to help pin this down. The gentleman I spoke to said he was passing it on to Spark to track down. The following day, Saturday, I received a text from Mercury stating, "Your broadband and/or phone fault has been resolved....". Though what the problem/fault was I am none the wiser π.
Since then the in-laws have not had any problems. π€
Big thanks for their help goto: Wheelbarrow01 from Chorus and to taneb1 from Mercury for expediting this matter. This is what these forums are all about. πβοΈ π
Cheers π·

