Apologies if this information has been posted elsewhere, but accompanying my recently received (and as yet unread!)copy of The 7 Ages of N,was a note from Howard advising that the revised Spotters Locomotive book was now available via his website ngaugemodelling.co.uk.
Or pick up a copy at TINGS.

I have many of the Spotters Guides but will probably not get the new one. I have:-

British Locos and other motive power 2020
Directory of N Gauge locos 2017-18
Directory of N Gauge locos 2015-16
Collector's Guide to British passenger stock 1967-2020
Collector's Guide to British freight stock 1967-2020

Those in conjunction with some old catalogues and the website N Gauge Resource get me through....

https://www.ngauge.org