Audi tt 225 - Battery Replacement

[EvoSlayer]

Any advice would be appreciated on the following.
The battery on my mothers 2003 tt appears to be on its last legs (lazy starting etc.) and I was wondering if an alternative/superior/cheaper battery from another manufacturer was available or should I just put an OEM part on again. Btw, how much do Audi charge for replacement batteries these days?

Cheers...

[EvoSlayer]

Thanks for that, I'll bear that in mind about the codes when I disconnect the old one. She'd be a bit cheesed off of she couldn't listen to Radio 2 anymore!