As GTSBoy said, while you have a ATMO Bov, the return pipe is not needed.
You can;
A) Buy some heater hose/silicon hose the same diameter and a welsh plug to bung in the end and tighten up the hose with hose clamps or
B) Same as above but if you cannot find a welsh plug a coke bottle lid works perfect or
C) I think Bunnings sell like a "rubber cap" which you put on the bottom of table legs to stop them gauging into wooden floor, not sure if they have the size you need (that's what I would the the original one on yours is)