Social worker.
Don't panic, there are (broadly) 2 sorts of SW: the children-at-risk-of-abuse sort and the children-with-disabilities sort, they usually work in completely separate teams. So you'll get the 2nd sort.
Social Care teams are seriously hauling back on the help they give atm and that may include setting tough criteria for getting an assessment in the first place. But afaik, pretty much any child with a serious/lifelong disability is a Child In Need (a legal term from the Children's Act) and they cannot refuse to come out and assess you if you make it clear that's what your child has. No guarantees you will get any help, or what sort it would be if you get it, but you should be entitled to an assessment at least.
Again, according to the law, there are 2 sorts of assessment: a core assessment (ie of what the child needs which in the case of a disabled child, usually comes out saying "child is fine because mum looks after them") and also a carers' assessment (ie of what you need as a result of having to look after them) and this is usually what decides whether/how much respite you get. Many SW departments do both in a single interview and write up one combined document, which is confusing, but you need to make sure your needs are discussed and documented whatever bit of paper they write it up on. If you know any other parents in the same boat (ie with kids with the same condition, or who receive the same support at school, or whatever) that's a big help because then you know what to ask for and what problems it's worth mentioning.
Ideally, the assessment should say you need x y or z respite/help and this can come in various forms. Whenever they can get away with it, SWs will "signpost" you to other services, ie avoid spending money by telling you to ask someone else who may or may not be helpful. Or they may spend their own money by giving you access to respite workers they employ (either within your home or outside it) or they may give you money to spend on employing someone yourself (called Direct Payments). Or a combination. The assessment may also refer you to other departments, eg Social Care have their own OT people who focus on adaptations to the house and environment, rather than school etc. We needed radiator covers and a high-sided bed but you may need a downstairs bedroom and an extra bathroom. It's painfully slow because Social care pay for it, but if you get a good OT it's invaluable, they can be very good and come up with solutions you never would have thought of.
Don't know if any of that helps - waffled on as always! Good luck!