How soon is this course? Might want to check out nannyjob.co.uk. You could even post on the chat thread. There might be someone in your area who is either between jobs or would gladly fill in some part time hours (if she works part time).
What about a babysitter in the area. If your mum is going to be there it doesn't sound like you really need to pay for a qualified nanny.
Maybe also post this question on the special needs section of MN. There may be some other parents of Austistic child(ren) who could advise what they've done if they've been in similar situation.