I haven't tried Seal Flex - if you get it I would be very interested to know how you get on.
Of the regular farmwear waterproofs, I find only the Flexothane and Airflex are in fact waterproof. And then only for one season. I bought a Delamare coat last year and was delighted with it for a few weeks, but then I started to be getting damp arms ...
The only actually waterproof, stay waterproof gear I have ever had was some used yachting gear. But even with those, the one suit had towelling lining, one had fabric elasticated cuffs and with both the water would seep into the cuff or sleeve lining and run back up your arms...
I have wondered whether the best solution is waxed cotton, have three or four sets of jackets and trousers, and keep reproofing. But then the last Barbour I bought (before I left the rat race, I get the make the market sells now!) leaked on first wearing...
What I most want to find is any maker / supplier of large LADIES farm work clothing. (Other than Barbour.) By the time I have an item of clothing that fits my ample backside, the legs need turning up a foot or two, shoulders come halfway down to my elbows...