From the 30th anniversary edition of Viz....
'Notes & Swearies with Professor Bernard F**k.
Dear Professor F**k,
Please can you confirm whether the correct spelling is "bell end" (two words), "bell-end" (with a hyphen) or the single word "bellend"? Only I need to finish this email I'm sending to Michael McIntyre.
THIS IS probably the question I am asked most often. I must receive this query from people who are writing to Michael McIntyre a dozen times every day - even more often when his Comedy Roadshow programme was on the television and, a couple of months ago, about three-hundred times in a single twenty-minute period following his appearance as the "star in the reasonably-priced car" on Top Gear.
The correct spelling is the two word version "bell end", but the alternative forms you posit are also perfectly acceptable usage in emails to Michael McIntyre.'