Mpuddle, get yourself into Wikivorce.com, there's an asset split calculator tool there that takes in consideration many different factors and gives you an estimate of what can you expect in terms of maintenance and % of assets.
There are a lot of things that could be considered when splitting, and although "clean breaks" are very popular now (I suspect because many couples have similar earning power and because most people look for a "cheap and quick" divorce which leave them in the dark of things they are entitled to), they are definitively NOT the best option if you are in a very disadvantaged position WRT your ex husband's.
But, as niceguy said, you have to consider what you are splitting, if the difference between getting 50% and 60-80% is less than 10-20,000 pounds, perhaps it is not even worth fighting.
BUT your spousal maintenance IS worth fighting, not because it is for you, but because it makes up for the meagre allocation you will get for the children. So get yourself the best solicitor you can pay, "The Which: Guide to Divorce", and fight for what belongs to you and more so, to your children.
It is not fair that so many men think that they can bless us with their children and then bog off paying a "symbolic" maintenance. Don't forget that if he has got to a good economical position in al these years you have been married, it is because also because you were supporting him and taking care of that family.