Thank you both for the reply.
After restarting the game I checked with the Warmaster and the Quest was now tagged as being completed. I was able to claim the Reward immediately with no problems.
Shadow, I was unaware that quests, once accepted & listed in the Log, you then decline and remove from your Quest Log would still be available from the giver. I imagine this doesn't apply to every Quest type since you specificly stated "If the Quest is still available.."?
"I cannot give you the formula for success, but I can give you the formula for failure, which is: try to please everybody." - Herbert Bayard Swope