I have almost cracked it on the 'cart' order template configureproduct.tpl:
The remaining problems are that you cannot pass the 'quarterly' billing cycle to the product any more (if you needed to), and also when the 'Update Cart' button is clicked, the first time it brings the page back unchanged, then a second click on 'Update Cart' successfully passes on to the cart view page with the price break down etc correctly showing.
EDIT:
I just noticed that now (using the above changes) when the selection is changed using the drop-down menu, it immediately redirects to the checkout page...which is no good at all.
Many thanks.