Can we just check that your nursery is charging you properly? By my maths, you're being massively overcharged!
Assuming both DC are in nursery for 45 hours per week each, that's at total of 90 hours per week to pay, and you've indicated that nursery charges £8.50p/h. That's 90 hours at £8.50 = £765 per week. Funding is 15+30 hours at £8.50 = 45 hours funded, so half the bill is funded, £382.50 - but your nursery is charging you £545...
I'm aware funding is term-time only, so your £382.50 only covers 38 weeks, = £14,535pa.
The other 10 weeks would be at full cost to you, so £765x10 = £7,650
I assume you and DH have at least 4 weeks holiday/nursery close so have worked out costs for 48 weeks.
Total care cost = £22,185, LESS £4,000 tax free childcare = £18,185.
If these calcs don't work, is it because I'm out of the loop in childcare costs (used it when DC were younger, monitored every penny), or because your nursery isn't applying costs correctly, perhaps?