Is my chimney leaking or is it the roof?

Assuming the lead flashing around the chimney is still in place and sealed/pointed we should be able to discount this as the source. Is your house a mid-terrace or is the wet wall a gable? If gable, is the pointing or rendering ect. in good order? and check the cement running down under/between the titles and the wall.There should be at least a 50mm overhang on the roof tiles. I wouldn`t go painting silicone waterproofer over the wall as it needs to breath and you don`t want to trap water in the wall behind the waterproofer. If it`s not the wall itself, then it could be the tiles, Have any slipped? Are any cracked, a hairline is enough. If they are concrete or clay with a roll or two, make sure they`re overlapping at least 65mm. Check also the interlocking edges aren`t broken and replace the tile if they are. If they are slate or stone there should be a row of half tile against the lower part of the chimney over the wall to cover the gap between the first whole row of tiles, often left out by dummies. The chimney itself should have good pointing or render and the pot should have a high steeply sloping cement fillet fully to the outside of the chimney wall, NOT flat at any point. If the chimney is no longer used, then it must be capped off or rain will penetrate down inside the chimney and make your wall wet that way. To cap off the chimney use a slate or tile cemented on, leaving gaps at the sides for air to circulate. so bed it on high-ish cement pile for a good air gap. When capping off a chimney you should also have vents inside the house at the bottom of each capped chimney breast to allow air to flow inside the chimney and prevent condensation, another often overlooked requirement, A small cupboard vent or hit n miss will be enough. Check to see the kind of underslaters felt you have, There was a period back in the 1980`s when it was all the rage to use a new strong untearable PLASTIC `felt`. Often light grey on one side black on the other. This turned out to be a disaster and caused massive condensation in the roof space, along with mould growth and rot. If you find you have this you will need to install plenty of venalation into the roof space to dry it out and prevent further problems. Replace it if possible with a new breathable fabric felt. Good hunting.

Drop the chimney below roof into the loft space problem solved it’s not in use

Get somone to have a look at it.

Looking at the chimney from the loft if the chimney flashing is worn you will see black water mark streaks diwn the wall. If chimney is not used try forcing a towel or cap chimney off and fit air bricks it could be the mortar that has failed and needs repointing