Yes they do do this, not to say that it is right, but they do!! I had this the first time round I dealt with them. I got the form, completed it and the next I know is a DEO with no further correspondence to me, when I investigated on their website they said it was used as a means of obtaining money where they have had difficulty, but they had not even provided me with an assessment let alone me refusing to pay.
You do right to wish to pay, and in fact whilst you are waiting you can do this on a voluntary basis and at the roughly correct rate as you can calculate this on their website which will ensure you do not get hit with a large arrears bill eg. the assessment takes 8 weeks with nothing paid then suddenly they want that 8 weeks returning in the space of 4 on top of your weekly amount!! Alternatively put the money you think should be paid to one side for now and when they ask for the arrears pay the lump sum straight off, but please do not bury your head in the sand thinking its okay for now as they have not asked, as the amounts add up from the first day of claim no matter what.
You can pay them directly over the phone (I believe at least) using your debit card, but speak to them and they will advise on best means of voluntary payment for now, just ake sure it is recorded and traceable no matter what!!
In terms of the change in offices it happens, and not always is it helpful in the future. I had letters from Falkirk, then Belfast, I would call one to be told the other was dealing with it, to go to the other to be told no it has been passed back, in fact there was a time where neither of them were dealing with it!! It is just another notch of their total incompetence, and I'm sure it will not be the first you come across. Anyway best of luck with it all.