MMP (who generally focus on diesels and other modern stuff) have introduced internal detailing kits for the various Slater's PO wagon kits. They are also going to produce brake gear in due course. Apart from being useful for detailing the legions of PO wagons that GC modellers need, readers of this blog will be aware that the Slater's kits can also be used to make GC hired coal wagons.
Being etched brass these kits should also add a bit of useful extra weight!
(Edit - although they are made of etched brass, the amount of weight added is marginal as the etches are (correctly) extremely thin. This should have been obvious to me, but wasn't.)