Its worth looking at the BAA website and maybe contacting one of the buddies
http://www.aerobatics.org.uk/training/buddy_main.htm near you for some advice. Aerobatics is a small world so they would know about any available shares.
There are some aircraft shares for sale here:
http://www.aerobatics.org.uk/aircraft+eqpt-for-sale.htm
Are you set on the Extra 300? It is quite an expensive machine to buy and to run. There are lost of cheaper ones and a share in a Pitts might be a better start. Many of these are on PFA permits and therefore very cheap to run. Insurance is not often a problem once you're over 100 hrs as they figure that you will be pretty careful as its your skin you are risking.
DJ