Problem passing plugin shortcode into CRED default value field

I use a plug-in which allows currency conversion. As an example of its use;

[currency amount="[types field="fee" id="$sell-list"][/types]" from="[wpv-post-author id="$sell-list" format="meta" meta="wpcf-seller-default-currency-code"]" to="GBP" show_from=false append="" round_append=""]

will convert the value in the types field 'fee' from the sellers currency into GBP.

I can use the plug-in in a view and it displays correctly. So a typical screen display will be '£41'

However, I need to find a way to extract the calculated GBP value and save it in another custom field as part of the action of a CRED form.

In my CRED form, I tried creating a generic field like this;

[cred_generic_field field="valuesterling" type="textfield" class="" urlparam=""]
{
"required":0,
"validate_format":0,
"persist":1,
"default":"[currency amount="[types field="fee" id="$sell-list"][/types]" from="[wpv-post-author id="$sell-list" format="meta" meta="wpcf-seller-default-currency-code"]" to="GBP" show_from=false append="" round_append=""]"
}
[/cred_generic_field]

Unfortunately something in the plug-ins formatting is causing the generic field to display as '<span style='' title='1 EUR = 0.8285 GBP'>£41</span>'

I've spent many hours trying to find an alternative way to store the currency conversion as a variable which I can store.

I'd appreciate any ideas you have.

For info the plugin is at http://wordpress.org/plugins/euro-fxref-currency-converter/

Many thanks

Robert