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.