If I sign up as a customer and take a PAYG phone , and assuming I keep the phone in credit, Do I have to top up to get the discount and if so how often?
See the Simply Broadband terms, here
http://www.orange.co.uk/terms/internet/homeservices/... (open PDF link and also follow link on the PDF and look for the phone fund PAYG terms on the page which opens, which means opening another PDF). Watch carefully when this "phone fund" allowance expires (Clause 3/9) but please check first!!
The Orange Broadband areas are, I believe, BT's 21CN exchanges, so if your exchange is still 20CN it's £10 more. This all raises a general point: read the Orange t&c's carefully so you know the small print! They're masters at it.