Years ago I tried to connect the Elei and Campus Elleti place-names. Here I wish to clarify (and emphasize!) that these two place-names are NOT, in fact, related.
This from Celticist Dr. Graham Isaac:
“R.J. Thomas (Enwau Afonydd a Nentydd Cymru,
[Cardiff 1938] 141) derives 'Elei, Istrat Elei'
c.1150 tentatively from *Eleg' + -i but offers no
meaning, while Ifor Williams (Enawau Lleoedd
[Liverpool 1945] 40) suggests that the root is leg
meaning dripping, slow-moving from which we
get llaith 'damp', cognate with Eng. to leak, and
However, on Elei, it would be from the same root
as Aled, Alun, Eleri, all rivers, < Celt. *al- < PIE
*h2el-, 'to shine'. They are all, in different ways, 'shining
rivers'. Elleti is not connected with these. The
form Elleti is corroborated by the instance of
'palude [Latin for “marsh” or “swamp”] Elleti' in
Book of Llan Dav (148). But since both that and
HB’s campum Elleti are in Latin contexts, we
cannot see whether the name is OW Elleti (=
Elledi) or OW Ellet (= Elled) with a Latin genitive
ending. Both are possible. My guess would be
that OW Elleti is right. As the W suffix -i would
motivate affection, so allowing the base to be
posited as all-, the same as in W ar -all 'other',
all-tud 'exile', Gaulish allo-, etc. Elleti would be
'other-place, place of the other side (of something)'.
There are certainly no grounds for thinking of a
connection between Elleti and Elei.”
Dr. Isaac agreed with me that the Elleti name, as he parsed it, would be the same as the Allitio (Alletius) name found (perhaps) connected with a god at the Corbridge Roman fort near Hadrian's Wall. Several dedications to Mabon (Apollo Maponus) were found at Corbridge.
The following source (which discusses the Book of Llandaff) suggests that the Welsh Elleti is to be found near Llanilid:
Ilid is supposedly an otherwise unknown saint. He (or she) was later misidentified with St Julitta.