Set of the three hammer springs for the Brocock Rifles these are around the 12mm outside diameter. Photo shows the  spring set, but this set ranges from medium to strong in power.
These are to fit the most common size Hammer Spring that is around 12.7mm outside diameter. But if yours is different then I can probably make a set up if you let me know the:
Outside diameter
Inside Diameter
Thickness of the spring

Set of three high quality hammer springs the Brocock pre charge rifles. Can be used when your old hammer spring goes soft or has broken. Can also be used to speed up the lock time and set the power nearer the 12ft lbs unless it is already on FAC you must not exceed the legal limit of your country. If you want the gun to be over 12 then apply for an FAC air rifle first. You need a locking cabinet and some land to shoot on. Not available for pistols.

We have found a lot of guns that come out of the factory a bit low on power. When the gun has low power it is cruel to go hunting unless the power is near 12ft. lbs.

Some new guns I have tested out of the box have been well below 10ft lbs, so make sure you check the power independently from the gun shop where you bought it. Also make sure the chronograph you are using is high quality and accurate. Some of the small cheaper ones are not very accurate at all. If you have a cheap one then check it against a good Chrongraph of good quality before you believe it.

First picture shows a set of three hammer springs, while the second shows just one to give you an idea.