[mdjlucan]

Where can I get onion sets for large onions thank you I live in London

[Salmo]

Anyof the commonly sold varieties of onion sets can grow big onions. The secret is to give them enough space. Space at 4 inches and they will grow to medium size. Space at 8 inches and they will grow bigger. Wider apart than that is probably no advantage. You need to plant them now.

They will not grow exhibition giants. To do that you need to grow special varieties from seed which needs to be sown in Jan/Feb. If you go on line to they sell plants of these varieties ready for planting out now e.g Marshalls sell Showmaster.

[fatbelly]

As Salmo says the bigger the space between each set will determine the Onion size.
I use a gap of 9 inches and get big Onions. I also plant then with a gap of 5 inches and get medium Onions. Sometimes the bigger Onions don't store as well as the medium size ones.
This year I planted them all with a gap of 9 inches and don't know why I did because I intended to plant with a gap of 5 inches but forgot 

[Salmo]

As Salmo says the bigger the space between each set will determine the Onion size.
I use a gap of 9 inches and get big Onions. I also plant then with a gap of 5 inches and get medium Onions. Sometimes the bigger Onions don't store as well as the medium size ones.
This year I planted them all with a gap of 9 inches and don't know why I did because I intended to plant with a gap of 5 inches but forgot 

You could poke another in between?

I have never measured but I imagine the total weight of onions per row is greater with the closer planting. Anyone know the answer?

[juvenal]

Onions planted closer produce smaller onions, but give heavier weight per row, according to Wellesbourne experts.