The Pineapple is the name of the ale-house to which John Childermass and Vinculus repair on the occasion of their first meeting. It is built over the culverted remains of the Fleet river, and indeed appears to be sinking into it, as the parlour is described as being three steps down from street level. It is an old building. A story tells of the death of its previous owner - a noted criminal - at the hands of an enemy who partly demolished the house to come at him, and though this was some time ago the house is still of ruinous appearance, the present landlord having discovered that its dreadful reputation is good for business[21]. Its ruined state may also indicate that the influence of the Raven King is yet to be felt there - all ruined houses, according to the old belief, belonging to him - and certainly it is there that a sort of manifestation occurs to John Childermass, when one of the Cards of Marseilles he is using suddenly and inexplicably acquires a look of the King himself.

The Pineapple must be within easy walking distance of Threadneedle-street, where Vinculus has his booth; although, if it is over the river Fleet, it will be some way to the west.
Fleet.jpg
The Fleet River, near St. Pancras, 1825 | A delightful steel engraving showing us what the Pineapple area might have looked like at the time of these Strange events.

For references to the fruit, see under Pineapples.
To see a 21st century interpretation of what happened in the Pineapple, please watch this little film (anno 2004) performed by Van Helmont.
Not exactly as Mrs clarke describes it, but a nice attempt nonetheless. I could be biased though...