My shed's on the boundary on my property, so one wall is only accessible from the neighbors property. The shed leaks from every side through the bricks, it's become a significant problem, and I need to have it fixed. It's very old, made of those big concrete bricks, the mortar's crumbling in places, though it has a new roof that I had put on a couple of years ago. Three painters have told me the exterior wall needs to be water blasted, patch primed, waterproofed, and painted - one guy suggested a rubber membrane.

The owner of the property (he rents it out) isn't very keen on letting me on his property as fixing the wall would destroy a mural/painting he had put there around 7-10 years ago, with permission from the previous owner of my property. He's tried to repair the wall himself but it still leaks in multiple places. I would prefer not to remove it, but the advice I've gotten is the best way to fix the shed is to strip it rather than say try to put a clear membrane over it. His suggestion is to paint the inside of the shed wall, though the advice I've gotten is that's just won't work, water will build up in the bricks and the paint will come off. If there is a way to preserve the feature that will reliably waterproof it for 5+ years I'd be happy with that. I want a permanent solution, I don't want to deal with it again in 5 years, and the painters all offer warranties - the rubber membrane guy offers 25 years. 

Obviously I want to get along with my neighbor, but I suspect we're not going to agree on this. I'm approaching it as gently as possible.

Do I need my neighbors permission to access his property? Any thoughts on the situation, legislation, the repair, etc?

Update: clear waterproofing product found, agreement with neighbor to apply reached.