The identity of Datchery isn’t difficult to fathom. Datchery is unacquainted with Cloisterham, so, at a stroke, every candidate is eliminated except Bazzard and Tartar. Bazzard is sulky and disobliging and clearly not the likeable Datchery, so Datchery has to be Tartar. (Additionally, Bazzard’s sudden disappearance from the text is too obvious a ploy, lacks ingenuity, and to have Datchery revealed as Bazzard — a minor, unlovable character — would be unsatisfying for the reader.)

As Rosa’s future husband, Dickens was bound to assign Tartar a major role in Jasper’s unmasking, and by making him Datchery that’s what he did.

Even Tartar’s name is suggestive. He’s a former sailor and the nickname for a sailor is tar, so the fact that he’s Tar-tar suggests a double identity, that he’s two men in one.

Furthermore, in Chapter 21 we learn that ‘Mr Tartar had a yacht’, making him a yachter. Well, rearrange the letters of yachter (as Mr Tartar himself did) and stick a capital D (for detective) in front and guess whose name you get?

In whodunits it usually appears impossible for the killer to have committed the murder. When the deed was done they’ve a watertight alibi. This is the method Dickens uses to throw reader suspicion off Tartar. Datchery is snooping about Cloisterham before Tartar has even heard of John Jasper. Or so it seems. In fact, between Chapter 12 and 13, Neville tells him the whole story.

The nature of their introduction establishes an immediate intimacy between them, and needing someone to confide in, Neville probably blurts out his tale at their very next meeting. (Note how quickly he relates his whole history to Crisparkle.)

Under ordinary circumstance no more would follow, but, by a highly suspicious coincidence, Mr Tartar just happens to be ‘an idle man’ in need of occupation. And the chance to play detective, and right a wrong, would naturally appeal.

But would he go to such lengths for a man he’s just met — one who could be mad or even a killer? Probably not. That’s why Dickens makes Tartar and Crisparkle schoolfellows. On hearing the name of Neville’s protector, and having established that this must be his old friend and master, Tartar would take on the job like a shot.

But why not reveal himself to Crisparkle? Because he was going as a spy and would be involved in deception. ‘False pretence not being in the Minor Canon's nature’ to involve him in the plan would be fraught with danger.

An additional clue pointing to Tartar as Datchery is that Tartar recognises Crisparkle in London. Is it likely that seeing a (hatted) man, not at close quarters, entering a house, he’d immediately identify him as a friend he last saw when he was just a boy of about eight or nine almost two decades before? Certainly Crisparkle doesn’t recognise him, even face to face and after prompting. It’s only after a pointed reference to their old relationship that he’s able to place him. As Datchery, however, Tartar would have made a point of sneaking a look at his old friend in Cloisterham and would instantly identify him again in London. (It’s mainly because of Crisparkle that Tartar adopts a pseudonym and disguise.)

The reason Tartar doesn’t make known his alliance with Neville, then, when meeting up with Mr Grewgious and the others, is that Crisparkle is too honest to be trusted. Furthermore, Neville — before relating his story — would doubtless have sworn him to silence. Tartar would also look forward to the pleasure of surprising Crisparkle at the end.

Nor is Tartar lying when he says he’s all at sea and doesn’t follow the conversation. He knows nothing of recent events surrounding Rosa or that Jasper is actively spying on Neville. In the conversation we overhear, Jasper’s name is never mentioned. Suspiciously, Mr Grewgious only refers to Jasper as “our local friend”.

Tartar readily agrees to Helena’s suggestion that he should call regularly on her brother — and why not? Little was to be gained by watching Jasper during the day. Only after dark was there likely to be any action. Making the trip back and forth between Cloisterham and London would be time-consuming, but Tartar has time on his hands to consume.

Dickens needs to show that making this daily journey is possible, so he specifies the journey time precisely. Jasper is to leave his London lodgings for Cloisterham, Princess Puffer is told, “at six this evening”, and is amongst the ‘arriving omnibus passengers’ at nine o’clock. A three-hour journey, then, twice a day. Perfectly feasible (by catching the first train, as Crisparkle shows, he can reach his flat at ten o’clock exactly) but it would keep Tartar fully occupied — which is why Rosa, after their first trip up the river, isn’t taken on others. Quite inconceivable if Tartar wasn’t being kept so busy.

It’s also noteworthy that on the day that Datchery meets Princess Puffer in Cloisterham, Jasper visits her in London. Why didn’t Datchery follow Jasper if he was keeping a permanent eye on him? The answer, of course, is that he wasn’t. Dick Datchery had already left for London in order to reappear as Tartar (whose Christian name, I suspect — why else keep it back? — would later be revealed as Richard).

A final point: the ‘Shadow On The Sun-Dial’ chapter was originally intended to precede ‘A Settler In Cloisterham’ (A shadow on John Jasper), so that ‘At about this time a stranger appeared in Cloisterham’ meant at about the time of Rosa’s meeting with Jasper.

Tartar tells Crisparkle that he’d met Neville “only within a day or so”, so the timetable of events must have been this: Tartar met Neville late at night, was told about Jasper next morning, travelled down to Cloisterham at lunchtime and returned that night. He then greeted Crisparkle next morning.

But is there a problem with this? After all, on the same day that Tartar met Neville, Crisparkle comforts Neville with the thought of his sister being with him “next week”. Yet when Datchery arrives in Cloisterham the following afternoon, Helena has already left. She could have departed that morning, but for this to be ‘next week’, the previous day must have been a Saturday. Dickens would have wanted Jasper to confront Rosa in the garden on a Sun-day, so it ties in with that, but is there any other proof?

Well, ‘full half a year had come and gone’ since Jasper showed Crisparkle the last entry in his diary at the tail end of December (probably the 29th) which suggests that the story resumes on July 1, neatly dividing the year in half. And when Christmas Day falls on a Sunday as it does in Drood, July 1 the following year is a Saturday (Leap Years apart).