Try here: http://search2.hmrc.gov.uk/kbroker/h...ormName=C+1331
You will see that yhou have to complete and deliver the one part to H M Customs before you leave and then deliver the second part after your return.
You should also have a Q flag to fly upon re-entering British waters at 3 miles out until you have telephoned H M Customs and obtained their authorisation to take it down. It is useful to have the customs telephone number (available from Form C1331) in your mobile.
How many people go through this procedure is open to question but bear in mind that you will be given a (much simpler!) customs form to complete upon arrival in the Channel Islands so that there is a means of H M Customs checking up if they are minded to do so.
Hope this helps.