On our Puck L it's the one over the kitchen window. It's got greasy so we cleaned it, but I am b*g*e*e* if I can work out how to tension it.
Any ideas please?
It'll be easy but as usual beyond my non practical mind

On the top left (but sometimes right) of the blind's cassette are two tensioning adjusters with slots to take a flat-head screwdriver - the upper rear one adjusts the fly screen, the lower front one adjusts the blind.

If the adjusters are obstructed by an adjacent panel or wall to fit even a short stubby screwdriver in there you'll need to remove the blind to make the adjustments, as explained in this article from Practical Caravan.

With both screen and blind fully retracted, turn each adjuster clockwise to increase the tension. After every half-a-dozen or so clicks pull the blind closed and check it will retract all the way up of its own accord - but don't overdo it - too much tension will weaken the spring.

John thanks, I think ours are different, very flimsy, but I took them off, cleaned them and rewound them and they are operating but not convinced the spring load is right. Will play again maybe tomorrow.