inventive

it depends on how the gateway module or script has been coded on your current website - for example does it send across the URL details or the name product model numbers of the products in the shopping cart during an order.

Depending on how the gateway has been setup and who the provider is, It is possible. However at some point they will find out what you are doing and you will lose the gateway. They will find out that gateway is being used on a website which is different to the the site when you signed up with them.
 
It is all based on risk, which is why the fees are higher. If you are in the UK give Payment Sense a call as they might be able to give you a better rate.

HI Neil,
 
I have just signed up

If you have to choose between Woocommerce or Opencart, then Opencart is the best one to go for.
Opencart is easier to install since you don't have to install Wordpress first. Opencart takes up less resources on a server when compared to Woocommerce and wordpress.
 
WIth Opencart the admin panel is more complex however this is because you will have more features to run your ecommerce website.
 
Opencart is also more customisable when compared to Woocommerce.
 
It is also worth your time to have a look at Loaded Commerce which is based on oscommerce and I have been using Loaded Commerce based carts for open 10 years (use to be called CreLoaded)
 

I have tested on Chrome, 22 inch Monitor , 1680 x 1050
 

The site looks good, big improvement over the old screenshot I have looked at.
 

Searching and browsing is ok, although the search box could do with being a little bigger.
 

The only thing I don't like is the download button being on the right hand side under adverts. It would be better if it was in the centre above the comment area.
 

Hope this helps