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.

I don't know what it is but just to say that black blotches will almost certainly be caused by a fungus rather than a pest and therefore treating with a pesticide is not going to help at all. Personally, unless the whole plant is at death's door I would ignore the blotchy leaves, which appear to be older foliage anyway, and just remove them if they really bother you. You could end up spending more money on solutions that the plant is worth.

MsOctober, your problem does not look the same to me. It does look more like a sunburn, where the leaves were wet midday. OP--it is hard to tell from the pic, and you don't give a location. Here in denver, basil is just not happy this year, because the night temps have been getting too low. I have 8 happy huge plants in an earthbox that have already given a couple harvests. But my in ground basils are puny, with small leaves and are only about a foot tall. That said, it does look as if you may have a fungal disease of some sort. I did have a potted plant with basil downy mildew, and the underside of leaf spores are very distinctive.

It looks like it might just be sunburn or nitrogen burn to me. I'm assuming you've not shown the undersides of the leaves because they look normal? Are you wetting the leaves when you water/fertilise? To be honest, I don't have much experience with diseases on basil, but I used to wet the leaves when I first started gardening and they quickly burned. It looks similar to what I can see in the photo.