I agree an express in full flow, or mollying along as my dad used to say, is impressive, but a heavy goods heaving up an incline takes a lot of beating.
Lots of spares on Peters Spares, have you looked. Here's an example. https://www.petersspares.com/hornby-x6599-class-8-duke-of-gloucester-cylinder-block.ir
I have always found Hornby to be very good, trouble people usually only post on here if things don't go well, pity. Really nice to see a positive response.
Don't get HO they look very undernourished and would have trouble lifting a shovel. The Dapol figures need painting and are easily adapted to represent different periods.
Dapol do a set of workmen with just what you want plus a few more. /media/tinymce_upload/dac453f597fca7e0fd7f832fb45e26f3.jpg /media/tinymce_upload/8cd05e9ed465c5b9b598f3dc888a5662.jpg
I just searched for Hornby BR green mallard and this was the 4th item. https://www.ebay.co.uk/itm/HORNBY-00-GAUGE-R2784X-BR-GREEN-4-6-2-CLASS-A4-MALLARD-60022-DCC-FITTED/283794466175?hash=item421377e57f:g:h50AAOSwaBJeVBrc
The line from Chinnor to Princes Risborough is only really a light branch line so I wouldn't have thought the locos woukd have been particularly heavy.
I'm not sure it is shoddy I reckon the detail is too fine for the size of model. There must come a point when you say enough is enough. Bit like adding eyelashes to an OO man. The detail on a large steam loco is easy to damage let alone something the size of Rocket.
When I did my GPO Telephones apprenticeship I soldered tens of thousands of wires and don't recall burning my fingers on hot wires. Did get hold of the wrong end of an iron once...........only once. 😉 You hold the wire against the track with the end of a wooden coffee stirrer.