I know nothing about repairing Polyethylene. I do have a repair kit supplied by Laser for the Dart 16. 2 methods of repair are mentioned. One is Rod Welding with a Leister gun as described above by John.
The other is Powder Welding. For this you need a hot plate and frying pan, a heat gun and a putty knife. And of course the powder which is supplied in this kit.
This is heated at 150 C for about 10 mins. The area to be repaired is cleaned and sanded - broken bits cut away etc. The area is heated, the putty knife is heated and the melted plastic is quickly spread onto the area. When cooled smooth with chisel or block plane. Finish by sanding and heat polishing the area. To heat polish: sweep the gun back and forth across the area until it glosses over.
That's what Laser say anyway <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />