That looks like an excellent website! Off topic, but involving buses off road, whilst searching I found a couple of fantastic photographs of coaches converted with flanged wheels working in the Hemel area too- a perfect model projectThere's a fleet history of their buses on the Local Transport History website (it opens as PDF so link is to the index page)

Many thanks Dave, excellent work there! You’ve gone the extra mile and more if you’ve spent several hours on it!Having little better to do today, I've spent a fair few hours online trying to solve this!
The bus is a Thorneycroft J, the distinctive wheels on solid tyres helped with that.
NK is a Hertfordshire registration. I gave up trying to identify it using the reg, so I've worked on the location.
I'm 99percent certain that our mystery bus went for a swim in the river Bulbourne, under station road, now the A4146, Hemel Hempstead here- https://maps.app.goo.gl/fgZ6T4eEaj7rEP5V6 (link to google maps image)
The bridge has been replaced, but the road remains to the next bridge over the canal, as does the tree lined path visible centre left on the photograph.
The real clincher was the distant steeple though. I'm pretty certain it is St John's the Evangelist, Boxmoor. https://maps.app.goo.gl/L1LozKpcURP5CtUp9 (link to google streeview) That view is from the canal bridge, but the building looks identical from that angle.
Now just have to work out the bus owner!
Edit- Allowing for the film and paper available in the 1920s, squinting at what few original photographs I can find, it looks like the LNWR crest on the side panels. Perhaps it was used as the 'Nickey ' line replacement?
HTIOU
NK 4558 was LMS fleet no. 2F, according to "Railway Motor Buses and Bus Services of the British Isles 1902-1933" Vol 1 (OPC, 1978) so this picture is presumably no earlier than January 1923, despite being in LNWR livery. (However, NK 4835, which is what the photo appears to show to me, is listed as 3F).Many thanks Dave, excellent work there! You’ve gone the extra mile and more if you’ve spent several hours on it!
Very much appreciated, as are all the replies.
Many thanks, I was wondering about the approximate date so it's good to have that information.NK 4558 was LMS fleet no. 2F, according to "Railway Motor Buses and Bus Services of the British Isles 1902-1933" Vol 1 (OPC, 1978) so this picture is presumably no earlier than January 1923, despite being in LNWR livery. (However, NK 4835, which is what the photo appears to show to me, is listed as 3F).
At this stage, the LNWR service ran from Boxmoor station (the current Hemel Hempstead) to Hemel town. Services to Harpenden ran from 1929, and to Whipsnade during 1931/32. All routes passed to London General - forerunner of the London Passenger Transport Board - in 1933.