Thanks, pretty sure it is the side seal but I hear what you saying about leaks. I had a t-roof on a.....well ya water can be sneaky.
The spacing looks even on the sides, to be honest I am guessing it would be better if there was less gap. I had a look at a couple of the neighbour's slides and I am not the only one where the wipe bunches, and it appears worse on the larger gaps. Not at all scientific, just surmisin. My worry now is the wipe is held in place by a row of screws and some rivets and if that is the adjustment then this looks like major surgery. I peeled it back and did not see any other obvious way to adjust it.