I don't think I can really do your questions justice. I would suggest as a first port of call the paper that describes how the Met Office generates CRUTEM and HADCRUT.
That paper is long and covers how the Met Office generates the trend data you see, how the data is modified and gridded, how error estimates are generated, etc.
Uncertainty estimates in regional and global observed temperature changes: a new dataset from 1850 P. Brohan, J. J. Kennedy, I. Harris, S. F. B. Tett & P. D. Jones http://hadobs.metoffice.com/crutem3/HadCRUT3_accepted.pdf
That paper is long and covers how the Met Office generates the trend data you see, how the data is modified and gridded, how error estimates are generated, etc.