On my 2011 Plus 4, there are just 3 main screws holding the dash, one central and one at each end, easily visible if you get your head down level. No need to disturb any leather. But deal with the undertray first...
The black plastic undertray is secured to the underside edge of the dash with several tiny screws along the length, so feel along with the fingers to get them all. The undertray stays in place with these screws out. The glove box comes out with the dash so empty that out first.
Some good advice from John above. You may find that as you pull the dash forwards, connectors may fall off and generally the loom might be rammed in like a load of old knitting! Mine was, so took the opportunity to tidy it up.
Don’t know what your custom part is but just be careful as all the press buttons etc are integral with the now somewhat infamous crappy, difficult to obtain printed circuit board!!
