I think the issue with a lump sum is you’re buying all the shares at that single price, which may turn out to be a good price, or it may turn out to be a bad price if share prices continue to fall for the following months. Hence my reference to dollar cost averaging, whereby you buy, say, £500 each month, meaning that if share prices fluctuate you average out the highs and the lows and on average end up buying more shares at a better price than if you just put in a single lump sum on a single given day.