Thank you Ray, that makes a lot of things very much clearer.
Since posting I did manage to find 2 references, I hadn't seen before, which indicate that WD198 got sent to Long Marston as 98 {I believe, as renumbered around 1968 time, although why the renumbering I am not sure}
http://www.wimrail.org.uk/mag/Corkscrew064.pdf about halfway down page 9.
and the other was a single picture that turned up in a search, of the 2 Long Marston locos, with Royal Engineer numbered clearly as 98. https://www.flickr.com/photos/22455491@N02/9861757194
I assume that it was painted back as WD198 when it went to the IoW, around 1991??.
http://www.iwsteamrailway.co.uk/RoyalEngineer.aspx
They all had lively and varied lives in their time and probably deserve more credit than is evident. {They were certainly good fun to drive.}
Thank you again for the clear information you posted, it is much appreciated.