There are some websites that offer detailing accessories suitable for Union Mills N Gauge Models. They include;

No photos, but here’s a quick explanation of UM locos.

Two screws under the tender. Front one secures the drawbar, and the chassis to the body. Body lifts off when screw removed. Rear one removes the baseplate - no need to touch this unless you want to change the coupling or clean the wheels one axle at a time. Motor is attached to chassis, does not come off.

Two screws under the loco. Front one fastens baseplate (which hold the wheels in) wheelsto body, and if there is a front bogie, also attaches the front bogies. Rear one fastens baseplate to body, secures the drawbar, and also secures the wire from the tender; this is important as it makes electrical contact so the wire is bare here. Sometimes the wire won’t come out when the screw is removed: if that happens, but I need to separate the loco and tender completely e.g for repainting, I cut the wire leaving a stub on the loco, then resolder the wire to the stub when I reassemble it.Body lifts off when both screws removed. Body is in one piece and does not separate any further.

When removing/putting back wheels, note that one side of every axle conducts and one insulates, even in the bogie is there is one (except the geared middle tender axle which is both sides insulated). All the axles in the tender need to be the same way round, and all the driving wheels in the loco need to be the same way round as each other, but opposite to those in the tender. If you don’t, you’ll get a short. The insulated wheels in the tender have traction tyres.

It’s easy (i.e. don’t do what I did!) to put the loco driving wheels upside down, they will fit. Look for the small raised parts on the coupling rods where they join to the wheels. These are the lubricating boxes and should be above the rods, not below. Sharp eyes (not mine) spotted that error in this D16!