At airfields with iap's, you should find that the met office will have provided a correction chart for the met observers to apply; there is not just a temperature correction; it will vary according to the 'range' of the observed pressure eg if QNH is between say 1003 and 1010 there will be one correction say 0.5 mb, whereas if it's in the range 1015 - 1020 it could be a different correction eg 0.8 mb
NB These are just examples; I don't have the correction chart in front of me.