I note that the LawnForce lawn fertilisers all have very different NPK content. How do I establish what is best for my lawn, I have been using LawnForce All Purpose with an NPK of 20-2-16.

• The fertiliser in Tui LawnForce All Purpose has high nitrogen that stimulates soil microbial activity and also stimulates root growth. This helps lawns develop a good root system that can withstand adverse weather conditions such as drought. Use in spring and autumn, it will feed lawns for up to 12 weeks. 
• Tui LawnForce New Lawns is great for establishing a strong root system when sowing a new lawn or patching an existing lawn, as well as rejuvenating lawns coming out of winter to spring and from summer to autumn. It has low nitrogen and is the only fertiliser suitable to use when sowing a new lawn and feeds for up to 12 weeks.
• Tui LawnForce Max Green includes added iron content if you're after a quick green up, use in spring and autumn, it feeds lawns for up to 16 weeks. For summer feeding use LawnForce Rapid Green liquid hose-on that is gentler on lawns in summer and for an instant green up, it feeds for up to 12 weeks.
• Tui LawnForce Weedkill includes a selective weed killer that eliminates common broadleaf weeds as well as feeding the lawn at the same time without harming the grass. It feeds for up to 12 weeks and for the best results should be applied in spring or autumn to actively growing lawns.