Paypal Payment Pro error

I have a problem with the confirmation page for paypal payment pro checkout. It seems to work fine, but the final page I get the follwoing error:

Thank you for your purchase.

Warning: Cannot modify header information - headers already sent by (output started at /var/www/vhosts/baselinesupplies.co.uk/httpdocs/shop/libraries/joomla/application/component/view.php:197) in /var/www/vhosts/baselinesupplies.co.uk/httpdocs/shop/templates/rt_crystalline_j15/index.php on line 26

I guess this is a template issue judging from the error? Or is it something wrong with my 'completion url'?

The error talks about two files, one from joomla and one from your template so it seems to be related to your template indeed.
You could easily hide it by adding a @ at the beginning of the line 26 of the file /var/www/vhosts/baselinesupplies.co.uk/httpdocs/shop/templates/rt_crystalline_j15/index.php

You could also check the link you provided. It seems like the problem is coming from Joomla. No transaction will be completed due to this case. See if all parts of your template are working properly.