OK, since I'm unable to sleep, I've done some more digging and found this on benefitsnow.co.uk:
"The criteria for living alone is that there are no non-dependants aged 18 or over who normally reside with you or who you reside with. A non-dependent is usually one of your adult children who lives with you.
The following are not classed as non-dependents and will therefore be ignored when awarding severe disability premium;
a person who lives with you and makes payment to either you or your landlord for rent" (my emphasis)
