Thanks for your response Tom.
Essentially there is only one query string variation that is live and linked to for the 10 pounds url.
/10-pounds-products/ => /10-pounds-productss/?display=cost&value=10
The text content is identical on both URLs however the query results are obviously different.
I don't think there is a way to remove the canonical tag aside from removing the plugin so I may have to go with changing it to the query string.
Ideally I would like for users to see the non-query version in the SERPs and the click through to the the query version of the page. Not sure that's possible now though I will speak to the developer to see if this issue is avoidable.
Thanks again
Rosh