I get the impression Chessington performs worse, but it's rising and has been since the addition of Wild asia.. Thorpe isn't rising as predicted, it's like it's reached a plateau after years of heavy investment, attendance rises and revenue rises. And that concerns them, because they put more money into Thorpe and it was going up and up and up... Thorpe make a heck of a lot on fast track, I suspect Alton do too, but Chessie do not...
Families definitely spend more than individuals. Except when it comes to stuff like fastrack, which gets expensive and unconsiderable when you've gotta account for more than just yourself. But they spend more on food, more on merchandise, more on entry, more likely to come by car so pay for the car park, etc.
There's a reason all the major destination parks choose families as their target market and all the teen-adult aimed parks have problems. Just look at Six Flags and Cedar Fair.
IMO, the best quality parks as the weird hybrids - the Busch parks, Portaventura, Phantasialand, Alton Towers, Dollywood etc. They target families, have varied, high quality attractions, but they don't skimp on high quality thrill rides either. Why? Because they know that if you spend big bucks on a major coaster that has presence (like a B&M), it'll be easy to market.
Look at Busch Williamsburg (old school, not so much now, they're being weird now) and Dollywood - their targeted marketing audience audience don't ride the coasters. So why do they have them? Because they're easy to market and are guaranteed to bring people in, whilst the overall quality and reputation keeps their target returning. The coasters are like boosts, not staples.
Herein lies Thorpe's problem. Their staple is coaster after coaster and an assortment of fairground flat rides. Their lineup is good, their assortment varied, and the insane speed of their growth made them fantastic for the greedy and impatient young adult audience. But in seeking that, they completely shut off the family market. It's wise to, whilst specialising, still provide for all bases.