The cylinder is capable of holding 4500psi, the regulator will output at a reduced pressure usually somewhere in the region of 600 / 650psi but regulators can be at lower or higher pressures (Up to around 850psi)
The Proto has its own inline regulator which again reduces the pressure to the desired level for the internals of the gun.
Have a read through this linked below, its a combination of the UKPSF HPA1 air safety form and a general guide to filling cylinders.
http://www.oaklandsfestival.host56.com/web_documents/air information.pdf
If you get the opportunity do an 'air pass' session at an event if provided. This gives you a basic overview of safety and how to fill. It won't make you an expert in compressed air but gives you what everyone should know.
At your local site they may or may not allow people to self fill, if they allow self filling then just ask for a quick run through of their fill station.
You can fill at either a 3000psi fill station of a 4500psi fill station.
(If you had a 3000psi cylinder then you could only use a 3000psi fill station)
For your own information look over the cylinder markings and the HPA information, you should be able to demonstrate that your cylinder is UK legal and in date.
Find your Pi mark (with the 'gate' Pi symbol, not the word Pi), the born date and the final date.
With a new cylinder you should see a recent born date (rarely the month you bought it as it has to get from the factory to a warehousem, the shop, shelf and then customer etc)
The cylinder is valid for 5 years from the born date, then you can get it hydrotested to cover another 5 years (you receive a test certificate and a label is stuck to the cylinder) and again at the 10 year point.
When it comes to the 15 year point at the end of the 'final' month the cylinder can no longer be filled.
