For a good while, we had a small leak from the upstairs shower into the garage below. I worked out that it was due to a problem with the tiling. Filling the tray didn't cause a leak but aiming the shower head at the back wall caused a leak. Removed the base tiles to find a huge gap between the shower tray and the walls. Filled it up, sealed it, retiled, grouted and checked to ensure the leak was resolved.

Today I find there's a larger leak - from three places in the garage. Not the shower walls, but if I fill the shower tray, there's a leak. Looked under the shower tray and can't see a leak. I suspect where the shower drain joins the sink drain is a leak of some kind. Filling/running the sink doesn't cause a leak.

Plan to call someone out as this is more than I can do myself but is there any way I can see where the leak is, or get at it?

Small amount of dye in the water will work but you'll need to clean it quickly afterwards etc to avoid staining

I'd guess at the rubber gasket where the plughole joins the waste

In the end, got a plumber out. We had a look and the waste pipes are fine. When I'd used a bucket to fill the shower tray, there was still water leaking because I hadn't left it long enough before trying the bucket. There's basically a leak between the shower tray and the tiles on a different side. Sod's frikkin' law. Sorted out one wall and now it's leaking somewhere else.

All this because my dad's a cheapskate and didn't want to rebuild the shower cubicle properly. It'd have cost £500. Instead he spent £250 for new tray and retiled cubicle, and I've spent more than another £250 on top sorting things out.

And the moral of the story is....