The above link gives the charges and does not say that disabled parking is free. However it has the following phrase, which may give hope:
Concessions
There are concessions available for certain categories of patients and visitors. Those with special circumstances and longer term parking needs should check with ward staff to see if they are eligible for concessionary parking.
Sorry I cant help you, your best bet is to phone them up and check. Different hospitals have different policies, for example of the two I use Bassetlaw Hospital has free disabled parking, Kingsmill doesn't.