Thanks...
There is a great number of archives there and I don't see anything which points me towards the one I should download.
I grabbed one and extracted it into /usr/local/share/picons and put that in 'Picon path'.
In the TVheadend GUI -> Configuration -> Channels/EPG -> User Icon (column) a file is referenced:-
picon://1_0_19_1_3_2268_EEEE0000_0_0_0.png
Is this the name of the picon which TVheadend expects to find? This particular file does not exist, so I may well have downloaded the wrong archive. Is there any way to tell if the above file is included in any of the archives?