I feel for you but you are only just back from mat leave and it's the beginning of term. Have your dc also just started nursery and so are unsettled?
I wouldn’t make any commitments about the long term yet but I would think hard about whether the childcare was right.
What time do you drop them off and pick them up? Are they in a day nursery or with a childminder? What arrangements have you in place for the inevitable coughs and colds that will kick in soon?
Can you maximise their time at nursery - dropping at 7.30 and collecting at 6 to exploit your working day outside the house?
Can your DH collect them from nursery one night a week so that you can work late to get ahead. Similarly can yiur DH drop them one morning a week so you can get in by 7.15 ish?
Can you sub-contract: cleaning, ironing, and have shopping delivered? Get into the groove of batch cooking on a sunday: dinner for Monday and freeze three weeks' worth. Then you always have in the freezer: chilli con, spag bol, fish pie, shepherd's pie, etc and you can dip in on your working days. Omelette, bagged salad, rotisserie chicken, soup and toasties- all life savers for speed.
I always found laying out/ hanging clothes the night before a lifesaver, and making sure bags were packed, shoes lined up in the hall etc.
More drastically can you book an extra half day or day of childcare for you? I know it's a ball breaker cost wise at this stage.
Radically, it might be easier to bite the bullet and go back full time. That remarkably lessens the guilt. You give 100% at work and work cannot try to pour four days work into three and at home you are realistic, accept you can't do it all and sub contracting is easier. I never worked so hard or felt more torn and exhausted as when I worked part time. I swear I did as much as the full-timers!
When your youngest is four, the au-pair route will open up if you have a spare room. That was a life saver. The cheapest form of child care and allows your dc to do their extra curricular stuff.
On the bright side, it's only six or seven weeks until half term! I'm guessing you must be in Scotland as you are back already?