For gas and electricity monthly direct debits are almost universally for the 30/31 days leading to the date of the bill. Thus a bill dated 1 Oct that you receive a couple of days later will have the (estimated) consumption though September.

And, yes, you are likely to have both bills in close succession - if you have been keeping up-to-date readings then this is not a problem but if there is a shortfall then this has to always be considered when deciding to switch.

And, remember, this is still 'summer' so the new bill may have an excess to balance out for a higher winter consumption and they may use a computer to guestimate your usage which may be totally out of whack with your actual consumption.

When you pay by D/D , is your pt on ( say ) 1 October deemed to be for consumption in Sept or an advance pt for October ?

Switched a few weeks ago , have had bill from Scottish Power till mid-August , new supplier has not asked for meter readings yet , will I have a final bill from SP and a D/D from new supplier in close succession ?

Stanley