A few questions

The onions aren't too bad but the yellow leaves on the courgette would indicate lack of available nitrogen or possibly magnesium deficiency - I'd try some Miracle Gro on the courgette (I meant squash). I know, I know, it's not organic but neither am I. I use artificial fertilisers sparingly as a medicine when I need to quickly sort a nutrient problem.

Not had blackfly on rhubarb - WOW! Actually I wouldn't worry too much about the blackfly, I don't think they do as much harm as some would have you believe if the plant is basically healthy and strong. Since we can't get Derris  anymore I'd try SB Invigorator which is organic and effective.

Adding some muck this winter will help no end for next year - do check the pH though, acid soils stop the plants taking up available nutrients. Lime part, muck part and in a year or two you'll transform the plot.

So good of you chaps, I'll follow all advice (can't use a hose on the site though) ... thank you so much, off now to do some 'squishing'.