I want to know if one can use a high pressure cylinder to store liquid nitrogen indefinitely and to then decant some off as a liquid for use as and when required. This rather than losing it over a period in a Dewar type low pressure container?

Liquid nitrogen is too cold to keep in anything other than a high pressure tank unless you wish to freeze yourself on the spot. Not recommended ever to just decant off some. What are you trying to accomplish with the liquid nitrogen?

It sepends on your appliction. High pressure LN2 is used to create lots of PPn2 nitrogen. Low pressure LN2 is used for cryogenics. Helium compressors can be a substitute for LN2, but can't for EDAX detectors etc because of other noises.

Twinkie, decanting off low pressure LN2 is done all the time to fill dewars. Not too hard to do.

PPE, long copper pipe, lots of ventilation. Watch out for stuck valves.

We converted all other uses but detector cooling to helium refrigeration cycles.

High pressure tanks of LN2 delivered by mistake aren't fun when your expecting low.