Apparently a few people on the Middle East forum were having the same problem and one of them recommended
https://getadblock.com/
I've installed it on my iPhone and so far the problem has been completely removed. Prior to installing it PPRuNe was virtually unusable, maybe 50-60% of forum pages loaded would get a redirecting full-screen ad requiring a browser shutdown - since installing it, no problems at all. Maybe there's something dodgy in the ads being served up on the forums.