My understanding was that the speed upgrade isn't from Pipex or the ISP at all, but BT upgrading their bit. You get the 1MB/s as soon as BT put it in.
I opted for the £15 one-off to get to 2MB/s, and it's brilliant!
I didn't realise Pipex have customer service - last time I tried to contact them (a couple of years ago) I got nobody on the phone but a machine telling me I was in a queue. I e-mailed, and got an autoresponse and nothing further.
However, reliability is very high, so I've been happy. I've not seen any cheaper offers from anyone with a decent rating on ADSLguide.