Kind of a sketchy discription of your problem.
I fertilize and treat my lawns about four to six times a year with small quantities. I never put out a large quantity as it can burn the grass. I also test the lawn before application of any fertilizer, Ironite or seed, since my neighbor works for USDA he runs the tests for me and I help with his yard. I would assume that you have a similar setup in the UK where you can get some sample testing to see what needs to be done to correct any imbalance in your lawn. This would be the best starting point.
Other things like aerating, overseeding, mowing and watering all have an important role over the course of time. Hard to be more specific without more information on your problem.