They should offer the minimum 15 hours - as getting less is disability discrimination. But as a private nursery they don't have to provide more than that. Not giving you notice is pretty poor.
Make a formal complaint under the nursery complaint policy - if it gets free places funding it must have a complaint and disability discrimination policy
You can ring the LA SEN Officer and Parent Partnership and ask if they can intervene / fund emergency extra 1:1 for the next 7 weeks to cover the 18 hours. LAs have a duty to ensure disabled children are not treated less favourably (Equality Act).
LAs can put some pressure on nurseries as they do give them SEN funding and also fund the free place - I am not sure what happens to a nursery who refuses to provide the minimum 15 hours. In theory a LA could refuse to accredit them for early years funding and pull the plug on their free places funding. In practice LAs are unlikely to do this as mostly there are not enough places. LAs can threaten nurseries under Disability Discrimination laws.
The LA will have emergency funding for SEN and can give extra funding to the nursery for extra 1:1. They may argue that anything over 15 hours is not education but private childcare arrangement, but they should fund up to the 15 hours
Under the Childcare Act LAs have a duty to provide sufficient childcare in their area including for disabled children. They should be able to give you info on alternative childcare but as they haven't given you notice / insufficient time practically that may not be much help. You could make the point to the LA if they are not helpful that they have a responsibility to ensure you do not lose your job due to their inability to provide childcare
You can apply to social services for an assessment of your child's needs and yours as a carer - which includes your right to go to work - and you may be allocated direct payments to cover extra cost of childcare etc. I would also ring them and ask for an urgent assessment. (see contact a family for a leaflet on assessments by social care)
Trouble is that wheels turn slowly and many LA staff will be going off on holiday for the summer so while this is really wrong, in practice unless you have an LA who will deal forcefully with the nursery or put in extra money they have not left you time to make alternative arrangements
you can ask work for unpaid parental leave but I agree that really sucks
Try ringing or emailing IPSEA for advice
I would also start to look into applying for a Statement of SEN with full-time 1:1 funding so you do not have the same problem when your child moves to the next placement