Las Iguanas restaurants have 'Happy Hour 2 for 1' cocktails available all day/evening throughout January. Found this out by chance when grabbing a quick bite there last night.
Alcoholic cocktails are £6.80 and the non-alcoholic ones are £3.20. Buy 1 and get another (same cocktail) free. Links in 1st comment.
Can personally recommend the Raspberry Cooler for those not drinking this month