Simon I find your explanations a bit weak to be perfectly honest regarding the trade rejections and here's why.
In the example where you lost £2000 because you coulden't hedge quickly enough because the price had moved,why don't you hit the ask straight away but go in with 60 or 70 contracts instead of the 50 ( £500 a point ). You will recover your loss within 5 points market movement max. ( or less depending on the number of contracts you invest ) Mr. Star punter A gets his price and you are making money within a very short space of time ( I take it he makes money overall betting at that level ) and lets face it he is trading by phone you have much more control over the transaction price than you would have online I doubt very much that £2000 losses are the norm. You are in the trade within half to 1 point of his entry ( you might even get the same price or utopia if it moves against him a tick or 2 and get in even better than him ) and everybody should be happy. If he keeps entering the market and the price shoots up each time and he exits very quickly before your losses are recovered then write him a nice letter closing his account and suggest he goes and plays with the large contract on the S&P via direct access,but really there shoulden't be any need for that.
The same principle can be applied to all your other punters.You mentioned the difficulties of selling into a falling market and vice versa then here is a suggestion as a hedge to counter that.
Each day start off in the market ( let's take the ftse as an example ) with 100 contracts long and short at the same time,you now have a neutral position which has cost you approx. £800 to set up in charges ( I would wager that you could negotiate better rates than that ). When Mr. Star punter A comes into the market with a long position at £500 a point you immediately sell the eqivalent number of contracts ( 50 ) plus what you want to risk yourselves as a company in order to start recovering your £800 transaction costs and make yourselves a bit of money,so now you have 100 contracts going long and now only lets say 30 contracts going short. In effect a position of 70 contracts ( £700 a point ) . This acheives the hedge,the punter gets his price, you get a better position instantly ( no £2000 losses now ) and you get to make some money by shadowing his trade. You then go back to neutral by buying the 70 contracts back when he exits.
You keep doing this for all your star punters and also when your book is unbalanced by £100,£200 a point or whenever your threshold is triggered. ( you don't necessarily need to add the extras for the unbalanced book. ). We all get our price,you get instant hedged positions and you can make some money off your star punters. Remember they are the best indicators you will ever get of market direction, if I ran a spread betting business I would love to have these big hitters betting with me,I would double thier stakes by hook or by crook but just get the money on,they are a gold mine.You close all positions at the end of the day ( if you want to that is ) and start again tomorrow the same way.
Why not let your dealers shadow all your winning punters this way,there really isen't any excuse for trade rejections or poor pricing and you get to make money as well and you still have your profits from a balanced book at the end of a day.
OK head back below the parapet wall now.