Yes, you need to declare it. But you can put your stock-related expenses against it. I use an accountant, but if you feel up to it, there's lots of information available from HMRC - the details and the forms change virtually every year.
You'll also have to fill in a form to get immunity from NI payments as your stock earnings are below a certain threshold. I found the NI bureaucracy far more of a pain than tax. Once you have filled in the form, send it recorded delivery and keep the chit.