Hi James,
And a very warm welcome to the forum.
A most splendid resource and with many knowledgeable, helpful folk.
And welcome to the fun and rewarding hobby of springer tinkering.
Personally, and as many above, I'd go for the standard spring. Sources for all the parts you'll need (some mentioned above) are Protek (yes, there's a spares section on there), John Knibbs, Chambers and TR Robb.
If you were ever to need to shorten a spring, there's an excellent article in the March issue of Airgun World on this very subject.