Bose

just chuck it in the boot

If you have a non Bose system then you can't really fit a Bose subwoofer without the Bose 6 channel amp, associated wiring loom and a lot of work splicing it into the non Bose loom. The Bose sub in the 8P A3 is literally just a different speaker in the same tupperware enlcosure you already have. The non Bose sub has an amp inside it which powers the non Bose sub and the rear speakers. The Bose sub is powered from the Bose amp behind the drivers side trim in the boot. To use this just to power the Bose sub, you would need to run power for it as well as tapping the feeds going to the existing sub to get a signal and remote on. You'd then have to extract the amp you already have fom the sub you are taking out in order to power the rears. A lot of work for a minimal improvement if any. You can't power the Bose sub from the non Bose amp as it will not like the 2 ohm impedance of the Bose sub and will die very quickly.
If you want better bass in your car then get one of my stealth subs, or a traditional boot hogger.
http://www.dizzythedog.com/index.php?f=data_audi_a3_stealth_sub&a=1

not sure if its just me being blind mate but couldnt see a price on your website for the sub and enclosure? also can these be delivered or do you require collection?

Delivery is £15, I've PM'd you the pricing.