Can CSA be reduced if kids are not in ex’s care?

I have 2 children with my ex, aged 4 & 3, and i have a 9 month old with my current partner. The 3 year old has a bowel condition which is pretty much living with Diarrhea 24/7. This requires a specialist diet, with specialists foods which cost a lot more.

She goes through more clothing and we go through more washing than usual when i have my contact.

She has been in and out of hospital since first being diagnosed at 5 weeks old. and lately shes been in there since the 14/01/15.

I have a 0 hour contract job which was doing ok but my hours have suddenly dropped to next to nothing.

Do i have to pay CSA towards that particular child whilst she is in a long term care at the hospital? as the ex leaves her there fri-mon (to do her washing apparently), the Hospital provide all food, nappies (due to the condition) etc. and 4 year old is staying with Maternal Grandmother the whole time.

Of which i dont get my contact, seperatly or together despite the temporary court order gives me 6 hours once a week.

So the question is can i get a reduction of my £180-odd monthly Payments because the children arent in her care, she stays at the hospital with her but only weekends and she has been in there over a month with more time expected.