Colorscheme vanishes after edit

Hi all.
Just upgraded from a VERY old copy of Cygwin (with a pre version 8 gvim) to a much newer version with vim 8.
I have a custom syntax recogniser for my primary language (VME SCL) which has been quite stable for the past however lomg it's been.
Now however, the colours al go away after an extensive edit on a file (forex ':%retab')
The file opens normally with all indents, folds and colours in place.
After the edit through, the only colours in evidence are on the closed folds and the 'fold depth' marker column.
If i place the cursor on a term and do '\hlt' it knows about the syntax item.
Has anyone seen this behaviour before? If so, any ideas on how to solve the problem?
Old system was an HP box running Windows XP (can't go back to it., the company trashed it!) New system is a Fujitsu laptop running Windows 10.1.
Output of the ':version' command appended below
Regards, Andy

VIM - Vi IMproved 8.0 (2016 Sep 12, compiled Mar 4 2018 23:26:14) Included patches: 1-1567 Modified by <cygwin@cygwin.com> Compiled by <cygwin@cygwin.com> Huge version with GTK3 GUI. Features included (+) or not (-): +acl +cryptv -footer +mksession +path_extra +syntax +viminfo +arabic +cscope +fork() +modify_fname +perl/dyn +tag_binary +vreplace +autocmd +cursorbind +gettext +mouse +persistent_undo +tag_old_static +wildignore -autoservername +cursorshape -hangul_input +mouseshape +postscript -tag_any_white +wildmenu +balloon_eval +dialog_con_gui +iconv +mouse_dec +printer -tcl +windows +balloon_eval_term +diff +insert_expand -mouse_gpm +profile +termguicolors +writebackup +browse +digraphs +job -mouse_jsbterm +python/dyn +terminal +X11 ++builtin_terms +dnd +jumplist +mouse_netterm +python3/dyn +terminfo -xfontset +byte_offset -ebcdic +keymap +mouse_sgr +quickfix +termresponse +xim +channel +emacs_tags +lambda -mouse_sysmouse +reltime +textobjects +xpm +cindent +eval +langmap +mouse_urxvt +rightleft +timers +xsmp_interact +clientserver +ex_extra +libcall +mouse_xterm +ruby/dyn +title +xterm_clipboard +clipboard +extra_search +linebreak +multi_byte +scrollbind +toolbar -xterm_save +cmdline_compl +farsi +lispindent +multi_lang +signs +user_commands +cmdline_hist +file_in_path +listcmds -mzscheme +smartindent +vertsplit +cmdline_info +find_in_path +localmap +netbeans_intg +startuptime +virtualedit +comments +float +lua/dyn +num64 +statusline +visual +conceal +folding +menu +packages -sun_workshop +visualextra system vimrc file: "/etc/vimrc" user vimrc file: "$HOME/.vimrc" 2nd user vimrc file: "~/.vim/vimrc" user exrc file: "$HOME/.exrc" system gvimrc file: "/etc/gvimrc" user gvimrc file: "$HOME/.gvimrc" 2nd user gvimrc file: "~/.vim/gvimrc" defaults file: "$VIMRUNTIME/defaults.vim" system menu file: "$VIMRUNTIME/menu.vim" fall-back for $VIM: "/etc" f-b for $VIMRUNTIME: "/usr/share/vim/vim80" Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_GTK -D_REENTRANT -I/usr/include/gtk-3.0 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/at-spi-2.0 -I/usr/include/dbus-1.0 -I/usr/lib/dbus-1.0/include -I/usr/include/gtk-3.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/ -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/pango-1.0 -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/ -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng16 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -ggdb -O2 -pipe -Wall -Werror=format-security -fstack-protector-strong --param=ssp-buffer-size=4 -fdebug-prefix-map=/usr/src/ports/vim/vim-8.0.1567-1.x86_64/build=/usr/src/debug/vim-8.0.1567-1 -fdebug-prefix-map=/usr/src/ports/vim/vim-8.0.1567-1.x86_64/src/vim-8.0.1567=/usr/src/debug/vim-8.0.1567-1 -fstack-protector-strong -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 Linking: gcc -L. -fstack-protector -L/usr/local/lib -Wl,--as-needed -o vim.exe -lgtk-3 -lgdk-3 -lpangocairo-1.0 -lpango-1.0 -latk-1.0 -lcairo-gobject -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0 -lintl -lSM -lICE -lXpm -lXt -lX11 -lXdmcp -lSM -lICE -lm -lelf -lnsl -lncursesw -liconv -lacl -lattr -lintl -Wl,--enable-auto-import -Wl,--export-all-symbols -Wl,--enable-auto-image-base -fstack-protector-strong -L/usr/lib/perl5/5.26/x86_64-cygwin-threads/CORE -lperl -lpthread -ldl -lcrypt "~/version.txt" "~/version.txt" 0L, 0C


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Andrew Long
Andrew dot Long at Mac dot com



--
Andrew Long
Andrew dot Long at Mac dot com

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