Dda and communal building access

[The Hulk]

Hoping someone out there can help.

My MIL owns a ground floor apartment within a large building (converted former brewery).
She is registered disabled due to decreased mobility,but as yet can walk a short distance without a wheelchair.
Her health is deteriorating and we are concerned the heavy entrance access door into her apartment building is causing her access problems,to the point where she has not been out of her apartment alone for 4 weeks due to struggling with the door on entry.

She will eventually need a wheelchair in the future and my question is where do we stand with regards to the building owners/management co (she pays a monthly fee for communal maintenance etc) with regards to the entrance door being modified for disabled access?
Are the required by law to provide suitable disabled access to the building?
My Dh has rang the management co today to discuss this,but they weren't much help and have merely referred this to a guy who lives in the building that deals with the maintenance etc.

Where do we go from here if this proves to go nowhere? Does the disability discrimination act cover this type of thing?

Any advice welcome thanks