AJAX HTTP error migrating large wordpress blog

I'm attempting to migrate a (reasonably) large wordpress blog from an exported wxr file into Drupal 7.8.

In terms of scale there are 369 categories, 193 blog posts and 6050 comments.

I initially tried importing the entire thing (without specifying drush path) but this fell over pretty quickly (same errors as below).

Instead I tried to import the following separately, but in the order below.

Categories.
Posts.
Comments.

The general behaviour is as follows. From the Migrate UI I check the item I'm importing, select Import from the drop-down and press Import.

For categories and posts, roughly half way through I get the following error:

n AJAX HTTP error occurred. HTTP Result Code: 403 Debugging information follows. Path: /batch?id=196&op=do StatusText: Forbidden ResponseText: Access denied | www.mysite.org @import url("http://mysite.org/modules/system/system.base.css?lrgvmx"); @import url("http://mysite.org/modules/system/system.menus.css?lrgvmx"); @import url("http://mysite.org/modules/system/system.messages.css?lrgvmx"); @import url("http://mysite.org/modules/system/system.theme.css?lrgvmx"); @import url("http://mysite.org/modules/system/system.admin.css?lrgvmx"); @import url("http://mysite.org/modules/comment/comment.css?lrgvmx"); @import url("http://mysite.org/sites/all/modules/date/date_api/date.css?lrgvmx"); @import url("http://mysite.org/sites/all/modules/date/date_popup/themes/datepicker.1...."); @import url("http://mysite.org/modules/field/theme/field.css?lrgvmx"); @import url("http://mysite.org/sites/all/modules/google_analytics_reports/google_anal..."); @import url("http://mysite.org/sites/all/modules/mollom/mollom.css?lrgvmx"); @import url("http://mysite.org/modules/node/node.css?lrgvmx"); @import url("http://mysite.org/modules/search/search.css?lrgvmx"); @import url("http://mysite.org/modules/user/user.css?lrgvmx"); @import url("http://mysite.org/sites/all/modules/views/css/views.css?lrgvmx"); @import url("http://mysite.org/sites/all/modules/ctools/css/ctools.css?lrgvmx"); @import url("http://mysite.org/sites/all/modules/panels/css/panels.css?lrgvmx"); @import url("http://mysite.org/modules/toolbar/toolbar.css?lrgvmx"); @import url("http://mysite.org/modules/shortcut/shortcut.css?lrgvmx"); @import url("http://mysite.org/themes/seven/reset.css?lrgvmx"); @import url("http://mysite.org/themes/seven/style.css?lrgvmx"); Skip to main content Home Hello admin Log out Administrative toolbarDashboard (View and customize your dashboard.) Content (Administer content and comments.) Structure (Administer blocks, content types, menus, etc.) Appearance (Select and configure your themes.) People (Manage user accounts, roles, and permissions.) Modules (Extend site functionality.) Configuration (Administer settings.) Reports (View reports, updates, and errors.) Help (Reference for usage, configuration, and modules.) Hide shortcuts Add content Find content Edit shortcuts You are hereHome Access denied Add to Default shortcuts You are not authorized to access this page.

I then have to 'reset' the item I'm importing and try again. I've managed to import all categories and posts, but with the comments the furthest I've got is 11% through (i.e. roughly 1000 comments) before it falls over as above.

I've tried providing the path to the drush command but that makes no difference. The problem is that even trying to pick up the comment import where it finishes gets nowhere at this point. Presumably it is to do with the volume?

I'm relatively new to Drupal, so was wondering if the problem could be config related - missing something in my php.ini perhaps, but the fact that I'm getting 403 indicates that it could be as simple as permissions, but I've no idea where to start with that! Any ideas