I know that the expense of shipping the Summaron to the UK might be daunting. But letting Malcolm Taylor have a go at it might be the only way to solve the problem--although he's not invincible, as I'll relate.
I bought a goggled Summaron on eBay for a very cheap price. Unfortunately, the goggles had been tampered with by some previous owner. They had actually epoxied the round prism in place--but, sadly, not the correct place. So they were out of alignment. Luckily, again for a very reasonable price, I found a set of goggles from an eBay seller in South Africa.
When these arrived, they were in good cosmetic condition, but out of alignment as well. I took them to my usual Leica dealer and he sent them off to his repair guy, who works regularly with Leica kit...three times. Each time the goggles came back in perfect alignment but, within a few minutes of testing them out, they were back out of alignment. It's apparently very difficult to get the prism in proper alignment and keep it there.
I finally sent it off to Malcolm. He kept the lens for a couple of weeks--he explained that the screws that Leica used in the lens originally were not always sufficient to hold the round prism in place. So he had to re-tap the screw holes and use slightly beefier and longer screws. Not only that, he said that once the prism was placed into proper alignment, he wanted to expose it to different temperatures to make sure that didn't cause it to come back out of alignment. Then, once he was satisfied that all was holding together, he would lacquer the screws in place and give it one last variable temperature test.
This time, the prism held--for about six months or a bit longer. It's drifted ever so slightly out of alignment once again, but not nearly so bad as before--likely because I am always bouncing the lens around as it goes with me everywhere. And even at F2.8, I get pretty good focusing accuracy, so I haven't sent the lens back to Malcolm again. Since it's a wide angle, stopping down a bit should cover any errors caused by the slightly out-of-whack prism.
Getting and keeping the goggles in alignment is indeed a tough task, and only a few people in the world are capable of dealing with it--as someone said earlier, even Leica regards Malcolm as being one of the very best. So if you want the problem solved as well as possible, it might be necessary to bite the bullet on shipping and go with him.