I can't say for sure in British gun terminology, but in North American terminology they are equilibrator springs. Basically you tension them until you have easy, almost balanced, elevation from level. Since there are two in this case, you will balance them left to right.
I was just involved in doing this with a M109 the other day, although they are equilibrated by hydraulic/nitrogen pressure.
If you need to know the length of new springs, I believe I have some at home, and there may even be one or two kicking around this museum.
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