Just been watching Scott's latest video on building your chest, i noticed he had his arms straight and locked before bringing his arms back in, so my question is should i be locking arms when my arms are strectched or is it ok to have a slight bend when the bar is away from your chest?

Definitely a big no no, locking them out would put stress on them, which can result in injuries. Just focus on the movement, squeezing the specs at the top.

A very slight bend is OK. But you want to come pretty close to full extension in order to get the most amount of muscle engagement :)