I assume you're referring to rolls of your spreadbetting FX position? If I have a plain cash position, I don't need to roll anything over.
For s/b positions, their volume is so tiny relative to the size of the FX mkt, they're irrelevant. Moreover, I don't think s/b shops actually physically roll over your position. I have a sneaky suspicion that it's just a way for them to earn a little more bid/offer. I could be wrong, though.
They will (or will not) happen the first time your trade settles.
Hope this helps.