Hi, any views on best replacement main spring?

The one coming out is a single unit, I see chambers sells :-

Mainspring-Airgun L/H & R/H Pair, MS017, Airgun Standard Mainspring, Pair (Flat section)

(with the above would anyone say a washer is needed between the spring sections? )

Are these any good?
Any other suggestions? cheaper?
Looking for smooth non harsh action, power not that important but up at 8-9ft/lbs I suppose is about right??
Cheers
John