Its simple... If they want Marshall's to be involved they have to pay. Marshalls must have had a competitive rate against the other companies like Serco in the first place when the first contract was ever put up for bidding. The fact that it is expensive to run air traffic control seems to shock alot of you. I wouldn't say that Marshalls have a Mr Grumpy and are being pains in the arse because lets be fair they arn't there for charity are they? They make money and run a professional organisation and have to be competitive against the other guys.
If they are digging their heels in its because they have lost alot of money and Sir P isn't making them confident that they will get enough of it back to be worth their investment. If they don't want to be involved because they are scared of getting bitten again then thats up to them. You can understand their predicament, you've got a chap who really doesn't have a massive clue who has less money or experience than the last investors and seeminly needs them to help it get up and running. They will probably then find they are on a short contract because once its going he will want to bring it in house to try reduce his costs further. In which case they are back to having lost money.