Quick pointer to this thread, where I describe how it's fixed for me:
Three is finally working properly OMFG.
70/30 Three unlimited SIMO, replacing 3 km ADSL line.
Previously: BT ISDN, Nildram, Plusnet, 186k, EFH, Be*, Plusnet (again), Pulse8, Sky, Plusnet Business, TalkTalk Retail.