from what I can see about this product it is one time application. it should have instructions on the container.
"EverGreen Complete triple-action - feeds lawns, kills weeds, controls moss. Visibly greener lawns in 7 days. With seaweed extracts. Evergreen Complete has extra Nitrogen for booosted greening power. Our unique N+ formula has been specially designed to deliver the best greening power. Hand-held spreader for quick application or for larger lawns the job can be made much easier by applying using an EverGreen or Scotts Spreader. For use on lawns only.
Contains MCPA, mecoprop-P and ferrous sulphate
NPK fertiliser 14-2-4" from: https://www.lovethegarden.com/products/lawncare/egcomplete.html
what do you think caused your lawn to be patchy? do you have a dog? I know they are the best at giving me a patchy lawn....lol I am in the process of reconditioning our lawn and it is a lot of work but I know it will be worth it.
You could try this in the patches: ( this is a snip from and organic lawn care guid)
remove any weeds, aerate the soil, reseed using an appropriate seed mixture and topdress with compost.
If more than 50% of the lawn is weeds, there is little organic material, the soil is very poorly drained or compacted to a depth of over 6 cm, it is better to remove the sod, work the soil deeply, amend the existing soil and reseed (or lay new sod).