So having just realised how untidy my cables all look around my headtube, my rear brake is hose is internally routed on the wrong side. How i haven’t noticed this in 18 months is beyond me but ah well. My plan is to disconnect it at the caliper, remove from frame, and re route it down the correct side. Before i render it as a display item, do i need new olives or any other bits to replace or is it as simple as wax on wax off? For reference its a shimano MT400 caliper in question

You will need new olive and barb. You may want some shimano oil and a bleed cup on hand to do a mini bleed afterwards.

I would suggest chopping the hose as close to the lever as possible, insert a screw each end then don’t forget to put the cover and nut on before reassembly

If it’s internal, it’s unlikely that you’ll get the hose through the frame with the compression nut’s attached, so you’ll need a new olive & barb (should be a brass barb on the 400, not the silver type). Personally, i’d remove the fitting at the lever, rather than the caliper, block the hose (sram’s red screw-in jobbies are good for this), and then you’ll only need to bleed the air from the lever, rather than the whole system.

The frame is completely open at the BB, so my idea was that it would be a helluva lot easier to route it down as opposed to up and out of the tiny hole at the top of the downtube

IIRC MT400 uses a standard barb/olive arrangement at the caliper end so you could go in either direction. Some of the other models use a banjo bolt at the caliper end so you can only thread the hose in one direction anyway.