I don't know if anyone is still having problems but this was posted over on the LupusUK forum:
"Hi all, if anyone is stuggling to get a slot, phone this number...0800 0525500, they were really helpful and will get you on the vulnerable list if you're not already. You will then have access to the delivery slots but keep checking as they only release 1 or 2 days ahead."
I hope it may help someone