Graham
I hesitate to contribute to this topic because my knowledge of colliery permanent way comes from visits to collieries in the 1960s and 1970s, with some 50 years or more of use and neglect from the period your layout represents. Also I'm not familiar with the Forest of Dean Coalfield or the GWR's arrangements for dealing with private sidings, so just bear that in mind in what follows. As an aside, I understand that the Forest of Dean Coalfield was excluded from nationalisation because of its tradition of free mining and any mines which came under NCB ownership were acquired after nationalisation.
I was rather intrigued by the decision to site the railway boundary to include the turnouts at the main line end of the full and empty roads. The actual connection to the main line would no doubt be carried out by the GWR and probably include a catch point if deemed necessary. That would subsequently be maintained by the GWR. Beyond the connection the remainder of the layout would be the responsibility of the colliery owner. Private Siding Arrangements were usually accompanied by a plan which detailed exactly who was responsible for what as in the following example
In this case the delineation between railway and colliery responsibility is conveniently at the railway boundary, but note that as part of the agreement the colliery company agreed to maintain part of their trackage to the railway company's satisfaction to enable railway company's locomotives to work a certain distance into the sidings and no doubt therefore the turnouts involved would be constructed to the railway company's standards.
However the junction between the railway company's and the siding owner's responsibility didn't always coincide with the land boundary.
In this example at the end of the branch from Parkgate Station on the Wirral, the colliery company's responsibility extends well into the railway company's land. In other examples the railway company took responsibility for maintaining track beyond their boundary. So the land boundary didn't necessarily dictate a change of responsibility or track construction.
This leads me to the question - how will the sidings be worked ? Is it intended for GWR locos to work trains into the sidings or for the colliery company's own locomotive to work trains into the adjacent goods yard. If its the former then the entry points to the sidings will obviously have to be to GWR standards, and also any track beyond which might be used by GWR locos in the process of dropping off or picking up wagons. The actual colliery track layout would be constructed by a contractor, maybe using some materials supplied by the GWR , so I would think its unlikely that they would change to a different track construction part way along the sidings. More likely they would use the same or similar materials, the only difference being that the turnouts at the far end of the sidings which would not be traversed by GWR locos could be of a sharper radius.
If the method of working is the latter and GWR locos will not need to enter the sidings the entire layout from the connection could be laid in lighter materials, maybe with flat bottom rail.
As for the question how were industrial turnouts constructed, basically the same as any other on the main line, the differences being sharper radii, maybe sometimes a slightly wider sleeper spacing and the use of ash ballast instead of stone. Colliery sidings were usually laid to a gradient under the screens to permit wagon loading without the use of locomotives. Ideally the entire layout would be laid at a gradient to minimise the use of locomotives, so that wagons could be run by gravity from the empties sidings, weighed on the empties weighbridge, loaded at the screens, weighed on the fulls weighbridge, and into the full sidings.
From visits to NCB sites in the 1960s-1970s, track was invariably chaired bullhead, i.e. the same as BR. I can only recall two locations where flat bottom rail was used, and which I recorded in photos - The Graig Merthyr system at Pontardulais in South Wales and the South Hetton to Seaham inclines where part had been relaid in FB. As for industrial PW being "disreputable", it certainly wasn't built that way although no doubt not quite up to main line standards. If it became "disreputable" it would be through heavy use and poor maintenance. Repairs were probably mainly carried out after a derailment rather than to prevent derailments.
Subsidence at rail ends was always a problem, with frequent dropped joints which should have been repacked, but this example is rather extreme at Haig Colliery in Cumbria. Quite worrying is that just behind the wagons is the edge of a cliff.
and NCB track maintenance wasn't always up to main line standards !
This very rough trackwork at Mardy Colliery in South Wales wasn't used by trains as it only led to the loco shed, but it was regularly traversed by the Peckett OQ Class 0-6-0ST known as the "Mardy Monster", one of the heaviest industrial locos in the UK, which probably b*ggered it up !
In contrast a bit better trackwork at Bickershaw Colliery.
I have more detail photos of NCB bullhead and flat bottom track which I can post if there's any interest.