Hi Toby
Yes dry soil is one factor but there is an amazing abundance of aphids just now, later than usual. I have some on tomatoes where ventilation is lacking in the tunnel and there are ladybirds too who are perhaps a little overwhelmed…
I would give the lettuce plants a fair soak, splashing the water on and off their leaves. It looks like the weather will help because rain is coming later on Tuesday and by this time next week we may have had too much rain and your aphids may be significantly reduced.