Hi everyone.
Having investigated many of the solutions in this thread I have identified a little cheat that has met my needs. It is by no means perfect but I thought I would share in case it is of use to someone.
I have some products that need to have the postage calculated based on order total and some as a flat rate regardless of order total. IE. some goods are products and others are gift vouchers with various prices.
I have used the standard shipping module and treated the weight feature as value. Where a product costs £169.99 I have inputted the weight as 169.99 and set up the shipping module in the same way, treating weight as a monetary value. Fortunately none of the products I need to calculate postage based on order total will ever be less than £10 so I have created a shipping cost based on weight 5 - 7 and where a product has a flat shipping rate, given it a weight value of 6
I have also encountered another little anomaly (just as I got excited to have found a solution!) that threw my theory out of whack! Don't forget to change pounds to kilograms in the unit of measure of the product ;o)
Regards
Laura