Jon, it may have been snow free where you are, but we've had snow on and off since mid January (south Sheffield, not too far from Lincolnshire where a lot of seed potatoes are grown).
Not bothering to sow or plant anything until April, hoping this weather will do one before then. Used DT Browns once before for plants, delivery was prompt & plants fine. Maybe they were difficult to source? Got mine from T&M on offer, but mine were all standard varieties, if you've ordered something special, maybe they are struggling to find it? Would be a bit disgusted and threaten to take my business elsewhere if I were you, that normally makes folk wake up & sort out the problem, with some sort of compensation (normally vouchers or free seeds/plants) for you. Hope you get it sorted out, this weather's got to pick up soon.