Hi Peter,
Love this layout.
In terms of shed inspection pit length details, the recent 'The Wigan Sheds Vol. 1 - Springs Branch MPD', by Chris Coates, has details of the rebuild in 1951 to the LMR depot modernisation programme and this shows up one or two details which may help.
The revised layout had a walkway across all roads approx. 1/3 along the length of the shed and the back half of the shed was capable of being closed off the full width with doors, the pits continued after the walkway but were also lit and interestingly weren't vented so presumably steam locos were shunted into this zone dead? The wheeldrop was also in the latter half of the shed, on road 6. I remember seeing the gantry rail for this peeping out of the shed wall when visiting with my dad to collect his wages in the early 80's (quick walk round the depot and then always time to catch a 40 whistle past on an ICI tank train [Hardendale tanks?]) all in the days before the roof was condemned, locos were banned from the confines of the shed building and the building being subsequently dropped. I've noticed a quirk with the Bachmann 4 road shed which has the pit roads lit (with dummy lights) and not the walkways - whereas all contemporary sheds of this type seem to show in pics that the walkways were lit.
Regarding Holyhead pics, the website of North Wales late 70's pics, www.2d53.co.uk, has a series of shed pics on the Holyhead page.
I've also got a book on LMR sheds somewhere which has plans for Bescot's 8-road steam shed, the fenced off shed was still standing last time a journey took me past on the M6, I'll root that out and see if the walkway strip was a regular detail.
Trust of help.
Regards
Ian
Note to self, you can't have a rear half of a shed which is trans-sected at 1/3 along the length by a walkway as that would still only be 5/6 of a shed??