I believe Presence of the Lord is an Eric Clapton song
In terms of value of the single I would say you have got a good deal. I have seen it exchange hands for substantially more than this though not so much recently. A copy recently sold on Ebay for five pounds. I was surprised it didn't go higher. Perhaps this is because the tracks are available on a 3CD "Rock Giants" compilation set?
http://www.towerrecords.com/product.asp?pfid=1520853
I don't think you could retire on the proceeds of this single. What you really want is the Popcorn Blizzard 45 from 1967. Now that might be worth a couple of hundred pounds?