Hi All,
I managed to sort this out in the end and just thought I'd let you know what I ended up doing.
I bought a Billion 7800 modem/router for each line and set them in Pure Bridged Mode. I bought the 7800 because I can tweak SNR values and they have Broadcom chipsets which is good for longer, crappier lines. I know they are overkill and a D384 would do the same but I know these modems are good for our conditions.
Now, the problem was, sourcing a decent bonding router and this turned out to be a real pain in the ass. On the one hand, I could have chosen a Cisco 2901 and an ADSL module for each line, but the retail value of this was over £2000. That's a VERY expensive broadband router.
What I chose in the end was a second hand WRT54GS from ebay for £20.00 - I had no idea until recently that the firmware on this, and many other routers, has been released as open source. Something to do with a software licensing cock-up, and this has opened the floodgates to a raft of hacks and tweaks.
The firmware I chose was Tomato/MLPPP, this firmware uploaded straight into the WRT54 and after a little configs - username, password etc, was up and running.
My only problem is that the ISP doesn't always route the connections through the same router at their end and this delays the second line from bonding. Can take up to an hour sometimes but I'm sure we'll get that sorted.
Anyway, after testing I can confirm that it doubles both up and download speeds quite nicely.
Just hope the above info helps anybody else in the same situation as myself.
Cheers,
Andy