GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: Experienced User

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Infuriatingly, when I add a new layer it appears but although transparent it is not seen until I roll the mouse over the mode to change it to something other than normal and change back again.

Any thoughts? It's driving me mad!. Restarting didn't help either.

Not seen where? Since it's transparent there is nothing much to see? Can you show a screenshot?

It actually appears transparent but if I paint on it it doesn't show and it hides all layers beneath it! Again changing the mode and changing back again works.

You are getting confused by alpha channel. If you have a layer with an image on it and you give it an alpha channel then you can select and remove parts of the background and make them transparent. (This is how to put people into new backgrounds.)

If it were that simple...however when I add a transparent new layer I do not see the layers beneath as I have been used to. Every time I have to change mode and change back again - very irritating. It's a s if I've changed a setting somewhere...

I saw this post about having an alpha lock enebled. I never use alpha lock so have no idea. I agree with Rich about the dark Gimp interface

Stupid question but is the eye icon showing next to all layers? Toggle it with shift + left mouse click.

What happens if you put a new layer at the bottom of the layer stack?
If you need to reset Gimp to defaults it's Edit > Preferences > Reset

Until we have a screenshot from the OP with 1) the Layers list and 2) the "New Layer" dialog this is just guesswork.

May, a day you did change something on this window when creating a new layer, check it out before to click OK

All very helpful. Many thanks.
In the end resetting preferences worked.