Would it not also be an idea to spray a seaweed solution directly on the plants? You should be able to get this in a garden centre (I can, here in Switzerland) or it can be ordered online at OrganicCatalogue.com
This could well be the quickest way to give them a general nutrient feed as it does not involve having to wait at least a couple of weeks before the “tea†is brewed or for fertliser applied to the soil to be broken down and then absorbed by the roots.
I also wonder if an internet search might provide information as to which nutrients are particularly low. I think I’m right in saying that certain deficiencies can give characteristic growth patterns / leaf colouration etc., knowing this you can apply an organic fertiliser that is known to be rich in that nutrient. As a rule, when using garden compost or well rotted manure you don’t have to bother with this level of research as all the nutrients are there already, but in this case it might be of interest, and if they don’t respond to all previous suggestions (highly unlikely) – maybe even useful!
One final suggestion (though I’ve never done it myself and I suggest it only if you are really keen to know exactly what the problem is) is to get a soil analysis done on the compost. These can be done through garden centres here in Switzerland, but I don’t know if that’s the case in the UK.
Best of luck, I’m interested to hear how you get on…
Compostpope