[welly]

This is just a guess, but looking at the 4.5.4 store I'm building, the file /skin/<yourskindirectory or common_files>/customer/main/product_details.tpl has the following line for displaying the price including tax:

<span class="product-price-value">{currency value=$product.taxed_price tag_id="product_price"}</span>

Ignore the bold type.

According to webmaster mode the untaxed price is held in $product.price, so you could try adding the following line somewhere in the same file, product_details.tpl:

<span class="product-price-value">{currency value=$product.price tag_id="product_price"}</span>

There in another line directly under the first one which (I think) displays the price in an alternative currency, but I have no idea how you might need to manipulate that if you also want to show alternative currency prices with and without tax...

Worth a try. It's where I would start if I wanted to display both prices.

Welly