Hello! I have just seen your post, so please forgive the delay.
Your piece by Purcell is actually by Jeremiah Clarke. It is very often used for weddings, so it can be found in many arrangements in wedding books. In that context it rarely is played beyond the first two minutes. For concert use lots of extra couplets have been devised by arrangers, an interesting one can be found here:
http://ks.imslp.net/files/imglnks/usimg ... pet-TA.pdfI am sure in your youtube clip Michael Murray's couplet in G major is improvised. It sounds more like William Byrd or other Elizabethan rather than Purcell or Clarke. The rest of Murray's interpretation is highly arranged, the thick harmonies are adapted to the organ and the acoustic. English organs did not have pedals for at least a century after this piece was written. It doesn't matter…
You can basically do whatever you like with this piece and people will still enjoy it!
You can find the basic tune at IMSLP.