I have a pea sized lump on my penis shaft (underside) which has suddenly appeared in the past 12/24 hours, it is not painful, what could it be?

Cyst: It "could" be a sebaceous cyst, which is not uncommon in the groin area. It could also be a small abscess - but that is usually painful. Depending on whether or not it is above the skin or under the skin it could possibly even be a wart. Without actually seeing the lump it is impossible to say. I would have it looked at by your Dr since it has appeared so quickly.