Thank you for your questions.
The wooden lintel needs to be a specific distance from the stove pipe and stove, not the hearth.
Wood, even fireplace lintels, is classed as combustible material, a wooden lintel should be at least 380mm away from the top of the Chelsea stove.
Flue pipes also get very hot, well over 300°C, so the lintel should have a line-of-sight distance of at least 380mm too.
Hope this helps?