Thank you for your question.
Freestanding stoves such as the Flavel Arundel don't usually give a distance to combustible materials above the stove.
The placing of a wooden beam on the chimney breast is unique in each case.
It depends on the distance from the stove pipe to the wooden beam which for a standard vitreous pipe should be at least 450mm from any exposed beam to the stove pipe by line of sight.
The distance can be reduced by using an insulated pipe such as Duraflue twin wall pipe.
We would recommend getting a Hetas installer to survey your property as they will be able to advise you and provide you with a "shopping list" of all the required parts for your new fireplace.
Find one new you here: https://www.hetas.co.uk/find-installer/
We supply a range of Geocast beams, which are made to look like wooden beams but are manufactured from fire resistant materials, you can find them here: https://www.directstoves.com/catalogsearch/result/?q=geocast
Hope this helps?