That certainly is not a lot.
One of the issues in all these disputes is that pay increases are always discussed in term of percentages. Give a post person a 5% rise, and they will be £1250 a year better off, gross, but only £1000 a year better off after tax.
But someone earning £50k a year will be significantly better off with a 5% increase.
Posties and other low paid occupations woukd be better asking for a lump sum increase, not a percentage.