mortgage is something which you're not in a position to change in the short term - for you possibly not even in the long term as presumably with 5 kids downsizing isn't going to be an option for you.
if you're with the cheapest possible utility company then there's not much you can do about that either, although yuu can look to see what energy you're consuming and see if you can bring that down. so i.e. turn down the heating by 1 degree. 1 degree won't notice that much - it's cheaper to wear a jumper instead and will save that bit of gass.
Make sure you turn lights off as you leave rooms. ditto with televisions/radioes/computers you're not using. Don't put the tv on stand-by, switch it off. Television on standby will use almost as much electricity as when it's switched on.
If you're buying value products already make sure you're actually getting value. Sometimes cheapest can be a bit of a false economy, because for instance if you buy cheap cuts of meat you're left with fat and bone, meaning you have to bulk out the meal more which in turn costs more money. Similarly with cheap fruit/veg - often the quality isn't as good and you can end up throwing more out.
Can you bake rather than buying some things? Things like cakes aren't necessarily that cheap to make, but making your own biscuits certainly costs much less than buying them in packets. similarly things like bread rolls - a bag of bread flour costs approx 97p and it will make you 30 rolls. A pack of dry yeast will cost you approx 50p and contains enough yeast for 60 rolls.
Look at other outgoings. Do you have a mobile phone? How much do you use it? is it contract or pay as you go - and if contract is the £20/30 a month worth it for the amount of calls you make/texts you send? If not switch to PAYG and top up as and when you need to. Also, if you shop around most companies offer good insentives for topping up which also save you money. so for instance I was with three. And for every text I received they gave me 2p cash top up, for every minute someone called me I got 5p, applicable when i topped up. so if in a month I had 100 texts and someone called me and spoke to me for 20 minutes if I topped up by £10 I would receive an extra cash credit of £3 based on the 100 texts and 20 voice minutes. Now I'm on orange and again you can choose things to have for topping up - I've put £20 on my phone this month and have received 600 free texts. I have until 25 December to use those, but that means that although I don't know 600 people to text, I won't have to pay for my texts this month, and next month I can just use the credit I put on this month, so £20 of credit will last me twice as long iyswim.
Also do your children have mobiles? Who pays for them and how much? Similar savings can apply there.
What about broadband? What speed are you on and can it be reduced? Most companies are offering sop speed broadband now, but reality is that most people just don't need that kind of speed, unless they're downloading films or streaming video (which most people aren't), so enquire how much it will save you to cut down to the lower speed.
What about television. Do you have sky/cable? If so do you have full package and if so do you use it? if not cut it back. so you have a package you do actually use.
What about your kids' school dinners? do they eat at school or do they have packed lunch?
Will try to think of more - hth