Right, first rule of debugging, ask the obvious.
Do your products in your catalog have weight ?
If you attempt to buy that EXACT shopping cart through another method, does royal mail work ?
Have you set up the percentage weight thing correctly in admin > configuration > shipping/ packaging >Package Tare... (I use 10%:0)
With so many unspecified variables, you try to narrow the search down to a cart that works, so even temporarily putting COD as a payment menthod, may get you to a stage where you can see royalmail working, then you repeat the steps with google.
Philip.
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