Hi Billyboy777.Welcome to the site.
‘Guyver’, I lived in the same area as your mum (Page’s Walk) but can’t say I can remember hearing that word used when I was there, saying that the way us cockneys talk at times that’s not surprising.
I have heard the word spoken but funny enough in Scotland (family) and Australia.
Your right with the meaning, very confident and arrogant, flattering talk, behaviour put on to impress. Today it would mean Bulls****r
There is another word that sounds like Guyver which we all used and still do, Skiver, Lazy person, a person not pulling their weight and is Scottish Slang for a Lazy person.
So two things,someone could have said Guyver and we thought it was Skiver? And second just for curiosity did your mum have any Scottish roots.
PS. Where in Leroy Street did your mum live?