looks like aphids
luckily to get rid of them you have a few options available
I would recommend using soapy water first
simply mix non toxic washing up liquid with water then spray all over the plant from top to bottom.
making sure you spray under the leaves as well.
the soap bubbles will suffocate the aphids.
if you also want you can use ...
I can see what might be black insects on some of the stems, but can't see them clearly. I can also see some white, oblong objects or marks on top of a leaf about halfway down on the left hand side of the second photo - again, its not possible to see them clearly.
These might just be aphids (blackfly) in which case it's worth spraying with an insecticide ...
It's the temperatures and light intensity as well as duration of light that need to be monitored.
Remember seeds are looking for moisture and warmth some look for light they don't need fertiliser until after the first true leaves appear usually after at least a week or 2 after germination
Don't use tap water in London it's too hard for most plants