A further update
The new part arrived. Here it is next to the original, you can see how it is more suited to extension, without the extra joint:
I then cut the brand new part in half, which just felt wrong!
The cross section of the cut has a diameter of about 17mm, which is too big for the 20mm OD 3mm wall tube I bought based on what I had read previously, so I'll need to get some more tube. Let me know if any of you have anything suitable you would like to sell.
A quick look on eBay found some 22mm OD 2mm wall tube which should fit. Will this be up to the job?