As I had to take the chassis apart to solder the brake hangers on (they were too close to the wheels for comfort) I opted to paint the chassis while everything was accessible. The frames were brush painted with some enamel primer and the a coat of Humbrol matt black, which amazingly dried matt. I then picked out the valve gear with some Humbrol gun metal to give a slightly shiny look of steel. A quick blast of acrylic railmatch 'frame dirt' has toned it down a bit.
Another task completed was to stick the balance weights on the wheels and fill behind the slivers of nickel-silver with some Humbrol filler. I'm very pleased with the way these came out.
I also primed and painted the underside of the tank and took the LNER off the tender.
Most remarkably it still moves when I put the wheels, gears and motor back in!