white and nerdy - I believe you can deduct payments for things you pay directly, such as school dinners. You may want to look at simply paying for all the school dinners and applying for a variation to take into account that you do that. The person on the phone won't know what they are talking about. Look at their website and write to them. I think a variation has to be something like a difference of at least 5% to the total though. Quick calculation on £300 a month comes up as 5% being £180. So you may have a case.
Sez are you amicable with your Ex? Have/Would there be issues over passports and him taking the dc away? Maybe he perceived this would be an issue for you and didn't want to cause a problem?
I mean he probably is an idiot, but maybe he was thinking that, or maybe he was thinking that having the dc at home gave them some kind of normality. You did after all say he had taken them out on a few nice outtings. Maybe he's trying to guage how spending a week with him and his new fg goes before going away somewhere together. Who knows!
And how much worse would you feel if he had said he was taking them to america or disney for a week, (something it sounds like you couldn't afford to do)? You'd probably be angry that he was doing that too most likely, buying them those things you can't afford too... So he can't win really now can he?
His sofa will get ruined too, don't worry about that. At least your home looks lived in and your house is full of love and laughter, whereas his is quiet most of the time...
Would you have agreed to your dc going to their dad's for two weeks? If so make it clear you are happy for this to happen and make it clear you are happy for him to take them abroad on holiday too. So he'll know it's ok next time.
And in reality I'm sure you would want some alone time if you had a new partner in the future. Would you never ever go away for a weekend together alone while the dc were at their dad's? He has a new partner, and hard as that is you have to accept he will do things with her without the dc.
I know it feels really awful when you see him doing things for her that he didn't do for you. It feels like you weren't worth it in some ways and that is a tough thing to deal with. You have every right to be angry and upset. But he didn't do those things for you, he wasn't prepared to compromise and you were unhappy together. You will be better off emotionally, and happier without him bringing you down like before.
It is hard but you need to learn to let go of these issues and remind yourself how much better off you are without him in your life. Of course he's the dc's dad and will be in their life. But try and separate the two and understand that you can't change or influence him and his decisions. Treat him like a 'friend' who may see your dc but who you wouldn't get angry or upset about, although you may get a little jealous sometimes.
Try and make him think about the dc and make it easier for him to have a good relationship with them. Be clear you are happy for him to take them away next time they go, assuming you are.
And next time he jokes about wanting his house back and counting down make your own joke about how they can come back to your lived in house where their presence is always felt and known because it;s their real home. I bet he won't like that.