It's pretty common for the payment to arrive before the award letter. There isn't a middle rate mobility but the first payment will take you from either the date stamped on the form (if you did a paper form) or the date you submitted an online claim, up to what will be your usual pay date, usually every 4 Tuesday's I think.
Thank you for your advice poltergoose - we got the letter today so all sorted. I'm shocked at what we've got to be honest (not that he doesn't need it ) as its been so easy - I thought we would have to appeal - perhaps I shouldn't believe all I read in the papers or perhaps I'm just so used to dealing with him and all that entails to keeping him calm and happy that it's "normal" to us.
It was only 7 days from receipt of application to the award. I suppose we have evidence dating back to his 2nd birthday so lots but I was still expecting a fight. A relief to be honest that it's been so easy - the last 4 years have been anything but. Thanks again