I'd do it now, Bornagain - the pots will be bent out of shape in a month or so if you don't, because the corms swell and get very large in a season.

Thanks Bamboo, multi purpose compost ok ?

It's what I use and they've been fine, but I can't help wondering if they'd do better in John Innes No. 2 - haven't tried it yet, but only because most multi purpose these days is absolute rubbish, it seems - though I bought some from Wickes, of all places, and I have to say, its lovely stuff, surprisingly.