Help: Dirty nappies and poo's dragging bedtime out for up to an hour!

[LazyDoll]

Hi,
I have 3 DS's. My DH puts the older 2 to bed whilst I bathe and bf the baby.
Bedtimes are taking forever for him as both DS's (4+8 and 2+10) do up to 4 poos from 1830 onwards everynight...
The eldest starts off on the toilet and is encouraged to sit for ages but will still be up and down 2-3 more times until he is finally finished.
His brother sits on the potty - with the same result - 2-3 dirty nappies before they finally stop and settle to sleep at around 2000.
HELP! What can we do to regulate the times they need to go (if anything!)
They always used to go in the morning (DS2) and late afternoon (DS1) now it's all saved up for bedtime but no change to diet etc.
Any advice gratefully received

My DSs (6 and 4) have been known to do this. I think it's mainly a delaying tactic - deliberately holding back so they can stall bedtime for as long as possible. I have no advice really, except perhaps having some kind of incentive to do a proper big poo in one go (eg for mine, it's 10 minutes playing on DH's ipod, which we introduced because DS1 was constantly soiling before he went to sleep in the evenings because he couldn't be bothered to go to the loo) however they don't get any more if they go back to the loo - it has reduced the loo visits but they are usually both up twice before they settle down - once for a poo, once for a wee. I'm contemplating ditching the ipod incentive, but 2 trips to the loo is far better than the alternative of DS1 not bothering with the toilet.