Hi Richard.
How do you lay out your invoices? I only show my 'trade' price in the non vat column just before the total of the invoice. The rest is listed separately and vat is charged on it. What exactly are they chasing you for? From what I have read in the past unless you are very careful they want to charge you vat on the whole amount. It seems that they are not happy with just the vat on the profit! I know of at least two other techs who only charge their customers the trade price to save them any hassle. I have had one vat inspection which was a long time ago and no mention was made at that time but perhaps they think it is another way to 'batter' us.
It seems that it is the only item that we have to disclose our 'trade' price to the customer.
Regards Colin