Unvented cylinder

Hi would just like some info on unvented cylinders I know you need a qualified installer but wanted to install the pipework myself. I am goimg to install a secondary return, would this be ok in 15mm from the last draw off point to the cylinder? Also I heard that for sink taps etc. you can use 10mm pipe is this correct. Any other advice would be great

I am using a 155L unistor and my water pressure is good in 25mm alkathine.

Get someone "on-board" first. but yes you can install the pipework on the whole.
10mm ok for short runs, maybe for a basin but not a sink.