Shower in another room leaks when bath is running
It may be due to a so-called 'power-shower': a little pump next to your loft tank that boosts the hot water pressure or just a well or thank pump that boosts pressure while water is in use.

If so, just check the gaskets in the leaking (hot-water) tap: maybe those are worn and cannot withstand the increased pressure or, if you have single-command handle, replace the inner cartridge.