I read the meaning of "theta sigma" in this page a few days ago (This page is very useful ^^):
link
And I also have found what you're asking about:
■The writing on the cliffs reads "HELLO SWEETIE ΘΣ Φ ΓΥΔϟ". ΘΣ (Theta Sigma) is a nickname used by the Doctor in the Time Lord Academy. However, since the the script quite clearly indicates the Greek letters are actually the co-ordinates to River's location in 102 AD England, it is possibly more instructive to view this as a sequence of Greek numerals.
Translating them non-additively, the numbers would be: Θ (9) Σ (200) Φ (500) Γ (3) Υ (400) Δ (4) ϟ (90)
An additive translation — the usual way Greek numerals work — would be: 9, 200, 503, 404, 90.
**Theta Sigma
"ΘΣ" was also a commonly-used abbreviation for "God" in Greek manuscripts of the New Testament. The capital letters Theta and Sigma together resemble the word "ΟΣ", (who), and some scribal variations in New Testament texts will have "ΟΣ" in a few passages while others have "ΘΣ" in the same passages.
I hope this helps :D
