Someone on here a few weeks ago actually put up the timings of the 'planned' schedule from STN to FCO three times daily and were supposedly be starting in March 2018 but since then it has gone quiet regarding AZ moving from LHR to STN.
If AZ are quitting LHR and looking to relocate to another London airport I should imagine that LGW would be a priority now that there are suitable slots recently becoming available due to the collapse of Monarch.
ch.aviation has an article where it states that any slots may not be of any monetary value due to the fact that Monarch no longer exist as an entity any more.