Thank you both for your kind comments. I have spent an enjoyable evening tweaking and adjusting the pony truck hornblocks and the compensating beam at the front end. I put the body on and pushed it around the tightest curves on the layout (about 3’) and first time one end didn’t want to play, hence the adjustments. I also slackened the coupling rod holes at the same time.
After those adjustments I pushed the loco, with body on, around the whole layout and it was very free wheeling and happy with the curves on offer. I’m not sure it would go round anything much tighter than 3’, so probably not ideal for industrial locations with tight curves.
I then had the bright idea of testing the gearbox (why didn’t I do that before I ask with the benefit of hindsight?!). The intermediate gears were pretty much locked solid so some careful lubrication and gently prising of the gears got it freed up. I am now fitting the pick ups, which requires some imagination because the mini frames holding the driving axle and gearbox takes up the space where I would usually glue some copper clad paxolin. The photos show progress so far.
John, I have a PDK Claud Hamilton to build and will definitely try flexichas on it. Mike Sharman recommended combining that with a hidden bogie holding the front two axles of the tender, then weighting the tender and using that weight to bear down on the rear of the engine to transfer some more weight onto it and increase hauling capacity. I will definitely give that a go.