Ok, so no good second hand unit available and can't afford a new one.
Seeing as this is apparently a common problem, this is how I have repaired my old unit.
1. Cut off the original rusted pipe flush with top of round plate.
2. Cut of about 2" of inner pipe flush with inside of round plate.
3. Drill through with 8mm drill.
4. Feed a new piece of steel 8mm pipe through hole.
5. Join to inside pipe with petrol pipe and 2 jubillee clips.
6. Seal around new pipe with Chemical Metal.
7. Coat of anti-rust paint and job done.
Don't know why I didn't think of this earlier