Flora, is the weather the same all across UK ?. Yes, seysonn - the climate is pretty much the same all across the UK with far less variation than in the US. Remember, many of the States are larger than our entire country. Essentially the majority of the country can be treated as one zone. In fact we don't even use a zone concept here since everywhere is roughly the same. It is quite possible UK posters have no idea what the zone box is for and wouldn't know their zone anyway. I had to look mine up but although it says 8/9 that really only fits for the minimum temperatures and gives no comparison with a US 8/9. There is nowhere here where it is reliably hot enough to grow large healthy outdoor basil plants without some sort of protection. Outdoor plants are usually runty and often ill looking. Some years it will do OK but it is a gamble. This year we have had (and when I say we I mean the entire country), the coldest least sunny spring and early summer for 50 years. We also have pretty much universally reasonably good soils so nutrient problems are not a common issue, especially for Mediterranean herbs like basil. Although, the OP's zone is not at the head of the post he/she said they bought the plants at Homebase, which is a UK store, and fed them with Baby BIo, a UK product, so I then checked their location and it said UK. Also I can usually tell by the language whether a poster is US or UK based, although sometimes the Canadians confuse me a little ;-) Bottom line is - basil, like tomatoes, is not suited to outdoor culture in the UK. It can be done but it is a project and is often a failure. Plus the OP did not separate the multiple plants. No amount of tinkering with feeding will overcome this. They need to be separated and protected. This is my basil - indoors in a container.

You can leave the basil in ground with a couple leaves and give it a shot--that may be cutting it back too far, but it's worth a try. Or, there's enough season left for a second crop, since it does grow so fast. Chives--yup, cut about 1-2" above ground (they're shallow rooted, so you don't want to pull on them). They'll keep coming and coming. Harvest the whole clump, or just a few leaves if you only want a little. Chives are perennial--you won't have to reseed them, they'll keep coming.

Basil is an annual in most cases. You can pot it and bring it inside for a while, but it's only meant to live it's life cycle out within a year and after a while it will go to seed anyway. There are a couple varietals that are perennial, but typical sweet basil is not. In addition--the hot temps and direct sun that basil prefers are generally not found indoors.

On years when I had excess basil I made large amounts of pesto and froze it flat in ziplock bags. When needed you can then just break a chunk of it, or use the whole bag. Another way to keep just the basil herb alone would be to process/grind the leaves, fill ice cube containers and freeze. Yet, another way is to dry it. Some basil dries really well.