i'm not LHR based - but I can answer your question.
Here's the full list of cruise charters:
16APR LHR-MLA-LHR
30APR LHR-VCE-LHR
14MAY LHR-GOA-LHR
06AUG LHR-XRY-LHR
20AUG LHR-ATH-LHR
03SEP LHR-IST-LHR
17SEP LHR-LCA-LHR
01OCT LHR-IST-LHR
15OCT LHR-ATH-LHR
29OCT LHR-LPA-LHR
..and yes - are all day returns!