Don't think I sent the below to the list, so resending:
Thanks Seth and James,
Things are getting a lot clearer. The BGP multihoming solution sounds like exactly what I want. I have more questions 
Now I suppose I would get my allocation from RIPE as I am UK based?
Do I also need to apply for an AS number?
As the IP block is "mine", it is ISP independent. i.e. I can take it with me when I decide to use two
completely different ISPs?
Is the obtaining of this IP block, what is referred to as PI space?
Of course internally I split the /24 up however I want - /28 for untrust range and maybe a routed DMZ block
etc.?
Assuming I apply for IP block and AS number, whats involved and how long does it take to get these babies?>
I know the SSG550's have BGP capabilites. As I have two of these in HA mode, does it make sense to do the BGP
on these, or should I get dedicated BGP routers?
Fixing the internal routing policy so traffic is directed at the active BGP connection. Whats involved here,
preferring one BGP link over the other?
Thanks again, I obviously need to do some reading of my own, but all the suggestions so far have been very valuable
and definitely seem to be pointing in some fruitful directions.
Adel