Every label says bright indirect light and I have no idea whether my flat has any, how do you actually measure it?

The variable that predicts nearly everything and that people ignore: distance from the glass.

Light falls off extremely fast as you move into a room. A spot two metres back from a window can be a small fraction of the light one metre back, and a fraction again of the windowsill. That is not a modest difference; it is the difference between growing and slowly declining.

So when someone says a plant is by the window, the useful question is how far, and the answer is usually further than they think.

What this means practically:

Move things closer before doing anything else. It is free and it is usually the whole fix.

Watch what is between the plant and the sky. A net curtain, a building opposite, a tree in leaf, a deep window reveal. The plant sees the sky, not the window: anything blocking the view of sky is taking light.

Clean the glass. Genuinely worth doing once a year and it makes a measurable difference.

Which way it faces matters most in winter. A south-facing window in July and the same window in December are different rooms as far as a plant is concerned, and this is why plants that were fine in summer decline from October.

A phone light meter app is not accurate in absolute terms but it is excellent for comparing two spots, which is all you need.

Your room is almost certainly much darker than it feels, and there is a reason that is nearly universal: your eyes adapt over an enormous range and plants do not. A room you can comfortably read in can be a small fraction of what a plant needs. Nobody can judge this by looking, so stop trying.

The free test that actually works is the shadow test. At midday on a bright day, hold your hand about thirty centimetres above the spot where the plant sits and look at the shadow on the surface:

• Sharp, dark, well-defined shadow - direct light. Too much for most things sold as houseplants.
• Clear shadow with soft edges: this is bright indirect. What most labels mean.
• Faint, fuzzy shadow you have to look for: medium light. A lot of plants tolerate this and grow slowly.
• No real shadow at all - low light. Survival territory, not growth.

Go and do this at every spot you have a plant. Most people discover that the position they thought was bright indirect casts almost no shadow.

Your symptoms - leggy growth, small new leaves, one that has stopped - are the classic signature of not enough light. Leggy in particular is the plant stretching toward a source it cannot reach, and it is unambiguous.

I grow well in a north facing flat, so the honest version of what that limits and what it does not.

What a north window gives you is soft light with no direct sun, all day. Right up against the glass that is genuinely bright indirect and a lot of things do well there. A metre back it is medium. Three metres back it is low.

So the useful move in a darker flat is not different plants first, it is different positions. Everything that wants light goes on the sill or immediately beside it. The middle of the room gets the tolerant ones or nothing.

What actually tolerates low light and still grows: pothos, snake plant, ZZ, aspidistra, some philodendron, ferns if the humidity suits them. Slowly, but genuinely.

What will always disappoint in low light, whatever the label says: fiddle leaf fig, most variegated anything, succulents and cacti, calathea in a dry room, anything with strong colour. Variegation in particular needs more light than the plain version of the same plant, because there is less green surface doing the work.

A grow light is a completely reasonable answer and it costs less than replacing plants twice. A single decent bulb in an ordinary lamp, on a timer for ten hours, turns a dead corner into a usable one - and the difference is visible within weeks.