[yocto] bbappend recipe causes errors with only two lines of code

[peterengcomau001 at adam.com.au]

I am using Yocto 1.6.1 and run a build that works fine.
I add a new layer and recipe
~/poky/meta-atmel-lsp/recipes-lsp/lighttpd with a recipe
lighttpd_1.4.33.bbappend and this has only two lines:
FILESEXTRAPATHS_prepend := "#{THISDIR}/files:"SRC_URI =
"file://lighttpd.conf"
This results in a failure:
/home/lachlan/poky/meta/recipes-extended/lighttpd/lightpd_1.4.33.bb,
do_installNot that the lighttpd.conf file is in the 'files' directory
as is identical to the original 
If I remove the line SRC_URI= ....... , the build works OK
If I now change absolutely nothing other than un-comment one line
(the mod_WebDAV module) in the local lighttpd.conf file:This results
in a failure:
/home/lachlan/poky/meta/recipes-extended/lighttpd/lightpd_1.4.33.bb,
do_unpack 
If I go back to the original lighttpd.conf file in my local 'files'
directory then do the following ->
If I change the second line to SCR_URI += "file://lighttpd.conf"I get
the same do_unpack error
If i change the second line to SRC_URI_append =
"file://lighttpd.conf" it also get the do_unpack error.
If I merge the two lines into one so ther is now only the one line
SRC_URI_append = "file://${THISDIR}/files/lighttpd.conf"I get an
error: Unable to get checksum for lighttpd SRC_URI entry
lighttpd.conf No such file found, and it refers to the original
recipe locations in
poky/meta/recipes-extended/lighttpd/files/lighttpd.conf
I get the same error using a single line when I uses SRC_URI =, or
SRC_URI += , instead of SRC_URI_append.
I run $ bitabke -c cleansstate  between builds. 
What could be causing the error. There is only two lines in the
bbappend file and the included file is identical to the original
.conf file.
ThanksLachlan
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