The original pipes have a plastic collar moulded around the pipes. If you very carefully cut through this, you can then pull the pipes off and be left with the fitting for the bulkhead ready for the samco pipes to connect to with jubilee clips on them.

Does that make sense? it's hard to explain but fairly easy to do if you take your time.

If it helps I can get close up pics of mine but I don't really want to be taking it apart.

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When I've got time I forget what I wanted to do, and when I remember what I wanted to do, I have no time!