Thanks to a suggestion from Nick Dokos, I have a backtrace:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  +(nil 12)
  font-spatial-to-canonical("+12pt")
  css-expand-length("+12pt" t)
  css-expand-value(height "+12pt")
  css-parse-args(238 257)
  css-parse("file:/usr/local/share/default.css" nil nil)
  w3-find-default-stylesheets()
  w3-refresh-stylesheets()
  w3-do-setup()
  mm-setup-w3()
  mm-inline-text((#<buffer  *mm*<2>> ("text/html" (charset . "us-ascii")) quoted-printable nil nil nil nil nil))
  mm-display-inline((#<buffer  *mm*<2>> ("text/html" (charset . "us-ascii")) quoted-printable nil nil nil nil nil))
  mm-display-part((#<buffer  *mm*<2>> ("text/html" (charset . "us-ascii")) quoted-printable nil nil nil nil nil))
  byte-code(" @A @bÆy`}ed|Ç	!È C\fA@)+4 \f@+> ÉÊ!\nË`,ÌÍ-\"c`ÎÏÐÑ?ÒÓÔDEEÕÔ\fDÔ+DÔ\nD-¯F./Ö0×1Ø-Ù2¯\f#ÚÛ,`ÜÝÞ3&	A@2Ë`,Ìß2§ à¨ á22@;¸ 2@½ â28@)#c`ÎÏãÑ?ÒÓÔDEEÕÔ\fDÔ2DÔ\nD-¯F./Ö0×1Ø-Ù2¯\f#ÚÛ,`ÜÝÞ3&äc åcj@;æ!b45q6)78ç!\n@d}èéGê9@!EâI9GS22@;Y2@^â28@)$,db\nÈ ¡)é" [ibegend handles begend preferred ihandles x 2 mm-remove-parts point-marker gnus-unbuttonized-mime-type-p "multipart/alternative" gnus-add-text-properties format "%d.  " gnus-callback lambda (handles) unless setq gnus-article-mime-handle-alist quote gnus-mime-display-alternative face keymap gnus-part gnus-data widget-convert-button link :action gnus-widget-press-button :button-keymap "(%c) %-18s" 42 32 1 (handles) "  " "\n\n" gnus-display-mime mm-display-part gnus-treat-article nil bufferp not-pref from id gnus-mouse-face-prop gnus-article-mouse-face gnus-article-button-face gnus-mime-button-map ...] 16)
  gnus-mime-display-alternative(((#<buffer  *mm*> ("text/plain" ...) 7bit nil nil nil nil nil) (#<buffer  *mm*<2>> ("text/html" ...) quoted-printable nil nil nil nil nil)) nil nil 1)
  gnus-mime-display-part(("multipart/alternative" (#<buffer  *mm*> ("text/plain" ...) 7bit nil nil nil nil nil) (#<buffer  *mm*<2>> ("text/html" ...) quoted-printable nil nil nil nil nil)))
  mapcar(gnus-mime-display-part (("multipart/alternative" (#<buffer  *mm*> ... 7bit nil nil nil nil nil) (#<buffer  *mm*<2>> ... quoted-printable nil nil nil nil nil)) (#<buffer  *mm*<3>> ("application/msword" ...) base64 nil ("attachment" ...) nil nil nil)))
  gnus-mime-display-mixed((("multipart/alternative" (#<buffer  *mm*> ... 7bit nil nil nil nil nil) (#<buffer  *mm*<2>> ... quoted-printable nil nil nil nil nil)) (#<buffer  *mm*<3>> ("application/msword" ...) base64 nil ("attachment" ...) nil nil nil)))
  gnus-mime-display-part(("multipart/mixed" ("multipart/alternative" (#<buffer  *mm*> ... 7bit nil nil nil nil nil) (#<buffer  *mm*<2>> ... quoted-printable nil nil nil nil nil)) (#<buffer  *mm*<3>> ("application/msword" ...) base64 nil ("attachment" ...) nil nil nil)))
  gnus-display-mime()
  gnus-article-prepare-display()
  gnus-article-prepare(2 nil)
  gnus-summary-display-article(2 nil)
  gnus-summary-select-article(nil nil pseudo)
  gnus-summary-scroll-up(1)
* call-interactively(gnus-summary-scroll-up)

It is
evident from this that gnus is using w3 to decode html.  However,
although I have w3 installed I have never been able to get it to work
reliably so I use w3m as my emacs browser.  I wish to use w3m to
decode inline html also.

mm-inline-media-tests includes the entry: ("text/html" mm-inline-text
(lambda (handle) (locate-library "w3m")))

which is set in my .emacs through the customization mechanism.

It appears that some other variable needs to be set: but which one(s)?

Thanks,

Roger Mason