At present, we have 5 ADSL Lines (Soon to be 8). I have a second 3300v which we were hoping to use in conjunction with the existing one, to provide an 8 way ADSL Load Balance / Fail over solution. Does anyone know if this is possible?

It should be possible to branch them pretty much infinitely, although that's kind of strange - the downside is you'd have lots of layers of NAT if you did that enough, however with one 3300 connected up with 2 ADSLs and that plugged into one of the WAN ports on the other 3300 which would have the other 3 ADSL lines and the original 3300.
The load balancing should still be able to work with that many connections

Still, if you can get that many WAN connections, maybe look into Fibre or VDSL services? Those are getting more common now and the speed difference is huge if you're not close to an exchange.

I am thinking of doing the same but using 2 Vigor 3300's to connect 8 ADSL lines at 5Mb each and feed the LAN from each into a third Vogor 3300.

I will then use the LAN from the third Vigor 3300 to feed my network with 40MB , Ok I know it will only give 5MB per client connection but they can have up to 8 connections if I allow it which I wont as I have up to 120 Client in total with say 10% online at any one time.

Before you ask fiber is not available so its this or Bonding and this is much cheeper.

Has this been done before or can anyone point out any bad points to watch out for.