Sapperfest is basically a big BBQ, camping trip, squadron smoker, madhouse and excuse for regressing back to your Royal Engineer youth. It's run by the Sapperfest Community Interest company, whose community is the Royal Engineers and the Royal Mons. It is ex-RE only - no wives, girlfriends, kids or other folks are allowed. They have a facebook group - https://www.facebook.com/groups/sapperfestofficial/
If it is obvious from this that you are ex RE you will be accepted to join. If there is no indication of service a message will be sent to you to give a history of your service and people you may know. It will help to have some of your service history on this site.
There is an official sapperfest website here - http://www.sapperfest.com
Hopefully someone will add the details of payment etc to the REunions page of this site so you can be kept up to date/ make comments/ ask questions there - but the basic info for 2015 is
CollopGateFarm, ManchesterRoad, Heywood, OL102NL - 10-11-12 July 2015