Truncated output with mod_layout/Apache 2

I'm using mod_layout to wrap (for various complicated reasons) the 
output of the Moveable Type editing CGIs, using the Debian package 
(4.0.1a) in conjunction with apache2 (mpm-worker, 2.0.53)

All I'm doing is:

	AddOutputFilter LAYOUT cgi
	LayoutHeader /cgi-bin/env.cgi

and the output truncates partway through - to be specific (for those 
familiar with MT) at line 87 in ./tmpl/cms/header.tmpl

I'm not using mod_gzip, mod_deflate or mod_cgid.

Anyone shed any light?