Theory:

I found Burlington House Nursing Home easily enough. There were paper chains up in the hallway and a lighted Christmas tree stood in the corner with a lopsided angel on top. I said I was a friend come to visit Mrs Macpherson to bring her a Christmas present. I could see through into the dining room where everyone was wearing a paper hat and singing. The matron had a hat on too and seemed happy enough to see me. She even offered me a mince pie. She walked me along the corridor. “Mrs Macpherson is not in with the others,” she told me. “She’s rather confused today so we thought it best if she had a good rest. She has no family you know, no one visits. So I’m sure she’ll be only too pleased to see you.” She took me into a conservatory with wicker chairs and potted plants all around and left me.
Explanation:
The author learnt that Mrs. Macpherson was in Burlington Nursing home and found it easily. The place was decorated for Christmas with paper chains and a lit Christmas tree in a corner, with an angel leaning to one side, on the top.  He introduced himself as a friend of Mrs. Macpherson and told he had got her a Christmas gift. He saw the dining room where the others were wearing paper hats and singing songs. The matron had a hat too, and she was happy to see the author and offered him mince pie, a traditional dish prepared specially during Christmas. As she walked along the corridor, she explained that Mrs. Macpherson was not with the others and that she seemed confused that day. So they had decided to let her rest that day. The matron told him she did not have any visitors, and therefore, she must be pleased to see him. She led him to a room with a glass roof and walls, with woven chairs and potted plants. She left the author with Mrs.Macpherson and left.
 
 
Meanings of difficult words:
  
Words
Meanings
Lopsided leaning to one side, lacking proportion or symmetry
Mince piea dish made of meat, dry fruits and spices traditionally during Christmas
Matrona lady in charge at an institution, a female nurse
Conservatory(British) a room with a glass roof and walls, attached to a house at one side and used as a sun lounge or for growing delicate plants.
Wickerpliable twigs, usually woven to make baskets, chairs etc
 
Reference:
National Council of Educational Research and Training (2008). Honeydew. The best Christmas present in the world - Michael Morpurgo (pp. 9-16). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.