for the first time this year i have planted onion sets.
they seem to be growing well but being a complete novice in the
vegetable garden could anyone advise me as to when they
should be lifted

This varies with vigour. Just lift a few and plant out or eat, taking notes of height of plant and results so that you have guidance for next year. Do not worry about making a few mistakes, learning to be a perfect gardener takes only 30 years. Do not let them start to form a flowering head.

Clearer thoughts. Let them grow until they start to ripen ( go brown), some time in August. Then tread down the tops along the row to ground level. This will stop leaf growth and energy goes to the bulb. Lift when the leaves go brown, late August/September. Store in cool dry place until required for eating.

thanks mondaychild

good advice , i will follow your instructions.
just for note the onions are already about golf ball size.
is this to be expected at this point in the growing season.

Golf ball is good.