Tony b,
As I’ve said before, the question is not are surface temperature records reliable? The question is, how reliable are surface temperature records? Your figure for the uncertainty range of “many degrees” simply does not tally with what we see in the data themselves.
Estimates of biases from the literature are typically of order 0.1degC and random measurement errors for ship data are typically estimated to be around 1 degC. The random measurement errors matter much less than the biases for large scale averages.
Even if SST measurements were subject to random errors of even 10 degrees, which they are not, then we would still be able to make a reasonable estimate of global annual average SST because individual random errors tend to cancel in the aggregate.
For more detail see here:
http://www.metoffice.gov.uk/hadobs/hadsst3/uncertainty.html
Best regards,
John