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Already cleaned, and relubed and it was moving pretty freely, but will check again.
@Jackytwoshoes, you can normally buy kits where cables are cut to the right length, but a few times shops have given me the wrong ones, as there are quite a few (e.g. all the different bar shapes, pre and post 2017 shifters and brakes).
I'm going to cut my own too. TheBromptonMan has a table of the lengths you need: https://www.thebromptonman.com/cable-lengths.html#/
infmz
sbbohr
There's a small hex bolt on the part which is located on the chain stay. Loosen that off, slide it along and clean out any crap which is in there then reassemble. Usually rectifies any shifting issues.