400 per month to invest

[Muddle200]

Stock market or fixed rate bonds

Depends.

What's your objective? Appetite for risk? Capacity for loss?

Can you make your question a little more vague?

Do you have any cash savings already? If not then you should build that first. Always in an ISA unless you've used all your allowance.

After that then the stock market makes sense but you need to be comfortable with the idea that you could lose money (although you probably won't in the long-run)

Well that is like ordering a drink and trying to make a binary decision between water and neat vodka!

There are plenty of options in between these two options ranging from lager shandy to Long Island Iced Tea's, to continue the analogy.

I'd suggest meeting with a financial adviser (not SJP) so you can go through a proper risk profile assessment looking at your objectives, appetite for risk, ability to absorb losses, and so on.