If you were to say shield a cavity with foil tape would it be of any benefit to connect the ground circuit to this, so that is becomes part of it, I am dealing with low voltage items here and not mains systems. Well it is a guitar to be honest and there is so much rubbish on the internet I thought I would ask more qualified and experienced people like yourselves who do not follow myths. I have what I think will happen in my head but just need to verify this.

Yes.

If you are shielding a guitar controls and pickups cavity, you connect it once deployed to the guitar jack cable connection earth, as a "floating" screen will do very little, and may even induce/ introduce even more unwanted stray pickup extraneous noise and "hum".
Of course when said guitar is connected to whatever amp, via the jack lead, and grounding is only as good as the amp grounding itself, and as nobody wants to run a separate "ground wire" from any guitar to any amp or even a separate ground, you get whatever improvements you get!
With electric guitars, you learn to live with most of the "noise" ones you can't eliminate.

Thanks I thought that would be the case and I just was second guessing myself as you do.

The 2" tape I have is supposed to have conductive adhesive, but I solder the edges together every 3/4"
Clean the edges with fine wire wool before applying, makes it easier.

Thanks for that tip, I would not have considered it until you said that.