Thanks for both the above. In answer to the questions - and for the benefit of other people who may have a similar problem now and in the future - my plan caters for 15 hosted domains, and I only have one currently.
Also, doom and gloom, I thought I had found the answer by transferring the "parked" domain to "my" plan - in fact that action appears to have caused the purchase of additional plan at some $239.40. This mistake was appallingly easy to make - no confirmation of purchase was requested, neither have I received an email confirmation, just a nasty little surprise lurking in the "invoice" section of my admin panel. :o(
Have written to Sales re: erroneous purchase of additional plan.
Will contact Support, as advised, regarding the domain that continues to sit in "parked" status.