Milltek decat without remap

A few weeks ago, I got a Milltek decat pipe for my S3 off a friend for a good price. I said I'd have it as long as he fitted it, as i know it's a ballache, but he'd already fitted it to his, then removed it again (for selling the car).
When he'd first fitted it to his, he didn't have it mapped for a bit, but said it was fine, but the engine management light came on after a while.
Mine's been fine for a while, sounds great, noticeable increase in power. I'd not driven it for a couple of week, as i've been in hospital, but when i got out, my mother came to pick me up, in my car. I drove home, but when entering a dual carriageway, i got up to about 4k, it hesitated, popped out the exhaust, then took off. At first i was thinking it's just been my mother driving it all ladylike when she's used it a few times, and i've got in and drove it harder and there's maybes been some unburnt fuel in the exhaust or something.
I've went out in it today, and it seems to restrict itself at 4k now, and it pops, but doesn't want to go any higher. Then the EML light has come on.
I'm hoping this is just because it's not been remapped yet, and the lamda sensors are picking up wrong emissions.

Views?

Not everyone has vcds thanks.

Sorry, i didn't mean to be abrupt in my reply. After speaking to a mate this afternoon, it turns out he has a copy of it, so i'm calling round tomorrow.
Also, I did always intend to get it remapped, but just had the exhaust fitted as part of the deal of buying it. Was going to follow it up with a reamp, but things happened that has knocked it back a bit.

Had the car checked on Vag-com today. Came up with an error of..

000801 engine speed sensor, intermittent fault.

My mate who checked it said it will be the crankshaft speed sensor, G28, and that I should find one on ebay for a fraction of the price of a genuine one an not have any worries. He said all his sensors all come from ebay and he's never had an issue yet.