Matthew, The inner seals, the one that goes right around in one loop, can be got from East Kent Trim. ( 01304 611681)
I have used them on several cars and they are just fine, cost is less than 20 GBP per side. I always remove all the horrible metal clips that are usually fitted near the bottom, and always make the joint near the door light switches, ( don't cut to length until you have all but the last 6" fitted) always hammer the new one on nice and steadily, but firmly, keep working your way around until it is definitely all hard down, then lastly, give several coats of silicone lubricant BEFORE you try shutting the door for the first time. I only did my last one last week and the door still shuts perfectly with no tearing on the new seal.
Chris P