You will have to pay Piscali again anyway, as there is a 30 day cancellation period, which they will insist on. You need to get moving with the MAC request (phone them in the morning!!), as you only have till the 29th to get the free month with IDNet, and Pipex will probably take their time issuing it, but by law, they have to issue it within 5 working days. You should just make it, providing you get the MAC by Friday, and sign up with IDNet on the same day. If Pipex haven't issued it by Friday, get on to them again, and threaten them with Ofcom if you have to.
Once you have the MAC, fill in the registration form on IDNet.com, and they will take it from there. It usually takes around 3-5 working days, depending on how quickly BT get their act together, and also depending on whether you have been LLU'd by Pipex / Tiscali, but IDNet will inform you of the migration date in good time. On the day of migration, all you should need to do is change the login details on your router or modem, and you'll be on IDNet. You can actually set up and use your IDNet emails before the migration, as soon as you have the registration confirmation and login details from them. If you are already on a MAX service with Pipex, it should be a straightforward switch, and you should immediately notice an improvement in speed. If you are on a fixed rate with Pipex, there is a 10 day training period, where the BT software works out the maximum stable speed your line is capable of. You may experience slowish speeds or occasional disconnections during the first few days of the training period, then it should settle down.
Hope that helps!