Andrew, the link you give on Ebay is accurate, and the price, for second hand a bit high, but it could be worse. New these are £300, without any attachments.
HDPE welding wire. Originaly I had a small cardboard box of Einhell HDPE welding rods Pricetag ‚20 for 25 30cm rods.
Then I just bought a rol of 5mm HDPE welding wire, from the same source as our full size HDPE plates. ‚30 for about 300m.

Hints for welding HDPE. Dont let it melt. If its been liquid, it becomes almost unweldable. Also, when working HDPE before welding, do not use any lubrificant, cooling or simular. This will make the surface equaly difficult to weld.