as per post, looking for 3 mm wall CDS tube that is 20 or 21 mm O.D. (i.e. so I can sleeve a 14 or 15 mm rod)

Anyone know where I can get a 0.5 m length?

Any hydraulic supplier.

Is THIS near you?

yes, Radlett is not too far away - I did see this tube size listed on another hydraulics company website in Scotland so those peeps may be able to supply it maybe .....

ebay?

"cfs steel 20mm"

but £10 including delivery so not cheap