On Sat, 29 Jul 2017 15:54:33 -0400, "tom"
wrote:

snip

p.s. How long is a piece of string question but ... Is it 'likely'
that diode just went (the charger is a good few years old now and has
been used quite a bit), or is it more likely something further down
the line took it out?

It is likely that the off line switch regulator is also failed.

There are two in this PSU so could that apply to one or both?

Grab a copy
of the TOP223Y datasheet to see the reference design for your supply.

I have looked at this:

https://www.digchip.com/datasheets/p...OP223Y-pdf.php

... and see roughly how it would work but not sure if that means I
should be able to test it somehow? FWIW, I have one of the little
component testers but I don't know if it will know what that is or
even damage it?

Here is a source for those chips:

http://www.ebay.com/itm/5PCS-TOP223Y...AOSwnLdWrEJ R

Thanks for that. However I am in the UK and but get them here also.
;-)

Cheers, T i m