My question is whether electrical work can be mentioned (but played down)
on
a general building regs application drawing and if this then satisifies
the
"notifiable" bit.

I don't know what you mean by "played down". However, notifiable electrical
work can (and should) be mentioned on the building notice application. No
further application is needed.

Can I cover these changes with general notes on drawing such as relocate
switches (for back room)
AND
relocate sockets and cooker point to suite revised kitchen layout (to be
determined)

I don't see why not. I would just say something like "Provision of
electrical services to BS6761 to new extension and existing kitchen area."
I'm sure they'd get back to you if they want more detail, whilst not going
into detail gives you some flexibility in layout if they don't.

Is it better not to draw attention to electrical work at all?

No. Do it properly.

Christian.