Got mine off Ebay, ready assembled complete with a shut off valve to isolate the air tank if required.
It has push pipe fittings and is a doddle to fit in the line from the valve block to the tank. (Looking from the front of the vehicle it is the first pipe coming out of the valve block, 6mm).
Depressurise the system, cut the pipe (make sure it is a clean square cut) and push the pipes into the fittings on the gauge with the isolation valve to the front. Make sure the valve is in the open position, start the vehicle and time how long it takes to refill the system. (7 to 10 minutes max). If it takes a lot longer, you have major leaks or the compressor is tired.
Compressor should cut out at approx 140 psi give or take a few psi.
You now have a way of monitoring the system pressure and isolating the tank for future work, e.g saves draining the tank if working on the valve block or Air Springs.
This is the link to where I got mine, you may be able to get one cheaper if you shop around.
RANGE ROVER P38 EAS AIR SUSPENSION VALVE BLOCK GAUGE & TANK PRESSURE ISOLATION | eBay
Hope this helps you