Free childcare hours and salary

Surfless

Evening all, 
I've managed to really confuse myself with my entitlement to free childcare hours, and thought I'd reach out for some clarity from the Hive Mind! 

Sitrep:

1 child aged 3yrs 3 months
Married, partner earns less than £100k gross per year
My paye salary on paper is less than £100k but due to changing jobs in late October  and collecting holiday owed as money not leave, I've exceeded the threshold by the end of April by around £4k.

Questions are as follows:

I've just realised the above to be the case, having already reconfirmed my details for April onwards earlier today. But in April I will have reset to a salary lower than £100k, so should I have spotted this in November when I last confirmed my details? If so how do remedy this? Can I ask to repay the 15 hours a week for January so far? Can I change it mid period to reduce further free hours being wrongly issued?

I read I can make a salary sacrifice into my pension to bring me back below £100k, but if I'm right that means I am going to have to sacrifice 4k across Feb and March pay days, which I can't afford....

What is my best option here? 

Thanks all in advance, this has caused a sleepless night.