How do I increase the tension in a garage door spring?

The FAQs on the Cardale site at

You can fiddle it a very little by cutting the spring, annealing the end, and reattaching. Other options are to add external springs, which may well be awkward.

Thanks but I'd looked there, it refers to instructions in a manual that I don't have!

That sounds a bit severe, I don't fancy taking the spring assy apart. I'd thought of locking the spring, supporting the door part open and then detaching the cables and winding them once more around the drum but hoped there might be an easier way(?)

Doesn't it also say that they should be on the back of the door? Maybe you've covered them up with your insulation!

On some doors there are alternative terminating points for the cables. Alternatively, you could add some. When all else fails fit an electric door opener(assuming this is possible) and the problem will not exist!

Regards Capitol

An up and over door normally has two tension springs, one on either runner, that you can wind up tighter with a steel bar or big screw driver. Doesn't your door have this arrangement?

No, there's a "clock-type" spring in the centre with cables going off to the edges of the door.

... nothing on the back of the door (even before the insulation went on).

Well, they have some contact details on their site - maybe you'll have to

This sounds like a cardale drum type spring,central in door frame aprrox 8" in dia

SORRY FOR SHOUTING HERE BUT UNDER NO CIRCUMSTANCES SHOULD A UNQUALIFIED PERSON ADJUST THIS MECHANISM,EXTREME DANGER TO YOUR EYES AND LIMBS WILL OCCURR AS HAS HAPPENED TO OTHERS WITH THIS DEVICE.

It is not designed to take extra weight to the door and I cannot stress enough the above advice,even professionals in the trade will shy away from these.Cardale no longer make these devices as the machine that wound them broke down and was uneconomical to repair.

You could assist the door by using a pulley and weight system/

And nothing obvious that looks like it could take a key or piece of metal bar? Somewhere in the centre of the spring housing I would have thought they'd give some kind of tension mechanism, or is it done by lengthening or shortening the cables?