Come to think of it, so far I've trimmed back our JA's (Fuseau) once they start flowering, but 15 ft still seems amazing.
Ours at 9-10ft are are probably the tallest but that may be because of Saddad's point: I actually clear the bed each year before re-planting in Feb-March but I suspect many others don't dig them out at all.
Also our allotments do have a bit of a hardpan about 2 spits down right across the site; it was originally scraped clear by bulldozers then after the allotmenteers protested some topsoil was brought in. Wonder if that's the reason?
On a separate point, some people on our site say that you should cut the tops off if they start to flower in order to stop energy going into the flowers that could be fed back down to the tubers during the die back phase.