Allotments 4 All

My onions are growing nicely at 100mm spacing and 225mm between rows. Can I plant leeks in the gaps? Eventually the onions will be picked and the leeks left to grow on, but will the interplanting affect the onions, or leeks too much.

Yes you can..you might not get the biggest 'show' onions but I cannot see why not... ::)..shouldn't say that..now they will come up 101 different reasons.. :-X ;D
Providing you can keep watering up and there is enough nutrients available for adequate growth....and bit thinning for early use..why not... ;)

Its a bit like putting all youreggs in one basket. If you get white rot in the onions it will probably affect your leeks too?

Interesting point about white rot. I normally put the leeks around the feet of the sweetcorn where they survive but do not develop into anything bigger than a thumbs width. Last year they were all affected by white rot. Is there anything that specifically causes white rot?

It's in the soil...
the best you can do is grow no alliums in the soil for the next decade or more...  :-X