Thanks Len and Brian for your responses.
In 1901 she is in fact included in the Wales census! I checked this a couple of weeks ago after another contributor to this board pointed out that some IOM records appear under Wales. Sure enough, she is listed as living with her daughter Ann Black and son-in-law John in Lezayre, aged 87.
Ancestry gives the date of the 1901 IOM census as 31 March. This would tie in with her dying and being buried on 7 April. Which census date is correct for the IOM (Lezayre)?
Also, the burial register states Lezayre but Brian's burial index has Andreas. Does this most likely mean she was buried in St Andreas church?
Finally, can anyone tell me if there is an MI? Thanks again for any help.